Why do we think dark matter is made of particles?

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Speaker 1: Hey, it' Jorhan Daniel here, and we want to tell you about our new book. It's called Frequently Asked Questions about the Universe because you have questions about the universe, and so we decided to write a book all about them. We talk about your questions, we give some answers, we make a bunch of silly jokes as usual, and we tackle all kinds of questions, including what happens if I fall into a black hole? Or is there another version of you out there that's right? Like usual, we tackle the deepest, darkest, biggest, craziest questions about this incredible cosmos. If you want to support the podcast, please get the book and get a copy not just for yourself, but you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters, and for the aliens. So get your copy of Frequently Asked Questions about the Universe is available for pre order now, coming out November two. You can find more details at the book's website, Universe f a Q dot com. Thanks for your support, and if you have a hamster that can read, please let us know. We'd love to have them on the podcast. Hey Daniel, do you think we'll figure out what dark matter is one day. Yeah. I have faith in our human intelligence. I think we're going to crack this puzzle. Oh wow, I didn't figure you to be an optimist. But do you think, like, if we ever find out what it is it might be like disappointing. I don't know. I don't see how that's possible. It's it's one of the greatest mysteries of science. Yeah, but what if the answer is kind of like not that interesting? Do you mean, like it's just a bunch of dark chocolate floating out in space and he had a lot of delicious matter out there. No, I mean, like, what if it's just like one kind of particle that does nothing interesting, Like it's a very simple particle. I think that no matter what the answer is, it's going to be satisfying to finally know after so many years of wondering. It seems kind of dangerous to have such high expectations. The universe has never disappointed me so far. Just wait till the end of this episode, though. Ye Hi am or handmade cartoonists and the creator of PhD Comics. Hi. I'm Daniel. I'm a particle physicist and a professor at U c Irvine, and I believe the universe has lots of crazy, fun adventures in store for us. Wow, you have a lot of high expectations for the thrill factor of the universe. I do, but it's also rooted in data. I mean, look at all the times we've discovered amazing, crazy, bonkers things about the universe. It keeps surpassing our expectations. It keeps even outstripping our science fiction authors in terms of creative ways the universe could be organized. Does that mean like every time you sit down and work at your office, it's like strapping on on a roller cluster ride. You're like, I don't know what's gonna happen, but it's gonna be an amazing thrill. Well, there's definitely always a question mark, you know, you never know in research if today is going to be the day you discover something incredible that changes history in the way we think about universe and life, or it's just gonna be another day of finding bugs in your code and you should have a seatbelt on your office chair. Justin king, I wear a helmet. Yeah, But anyways, welcome to our podcast. Daniel and Jorge explain the Universe a production of I Heart Radio and welcome to the roller coaster ride that is the discovery of our universe. We have just started to crank on up the tracks. We are that part of the roller coaster. We are gaining elevation, and you know that thrills are coming because most of the universe remains a mystery to us, and our job on the podcast here today and every day is to ask questions about all of those mysteries, to wonder what else is out there, how does it work? And are we gonna lose our hats when we find out? That's right? Don't raise your arms, wint in physics. It makes it more exciting. Do you think the roller coaster physics would be like a series of couches trapped into some wheels I see with physics taking naps on them. Yeah, there you go, and it just goes really slowly and slowly upside down and then the coffee spills out. You don't sound very excited. Don't forget that there are big adventures in physics. There are times in our history when we've had to throw away everything we thought we knew and completely reorient the way we think about the universe. So I don't know if those are drops or twists or turns or skeletons jumping out of the hodded house, but there's definitely some thrills on this ride. Yeah, it is a pretty interesting and amazing universe, full of surprises, and it's also full of giant, enormous mysteries, like, for example, the question of what is the universe made out of? That's right, We've been spending a lot of time trying to think about what we are made out of and what you are made out of, what that rock over there is made out of, what the rock himself is made out of. But it turns out the most to the universe is not made out of that kind of stuff. Most of the universe is something weird and different and new we only recently discovered even exists. Yeah, it's not just a little bit of the universe. It's a whopping third of the universe. It's something we totally don't know. And it's not just a third of the uni verse. It's like five times the amount of regular stuff in the universe, the stuff that you and I are made out of. Yeah, and I love how we call our stuff regular stuff, even though it's the unusual stuff. You know, it's it's the weird stuff in the universe. Most of the universe, the matter in the universe is this other weird stuff that's called dark matter. We are the sprinkles on top of the cupcake. We are not the cupcake. I guess I'm in regular as in, you know, like your bowel movements, boy, I mean, the rest of the universe is constipated. I don't know if anything's gonna be associated with bowel movements. I think it would be dark matter. Yeah, because it's dark. You know. My wife studies the biological dark matter, which is most of the stuff in the human genome and in the viral genome that we don't know. So it turns out that there's lots of things we don't know about the universe inside and outside of us. Yeah, they say a big part of our biology is a sort of unknown. And also just the very things that the universe is made out of. I mean not just us and you and me, but like all the rocks, all the us out there, all the dust, every planet out there is mad at of stuff that is not common in the universe. That's right, and that's unsatisfying, right. We want to know what the university is made out of, which means we want to understand everything that's out there, not just a tiny bit. If you wanted to study elephants, you wouldn't just look at their trunk, or you wouldn't just take a close look at your ear. You want to understand the whole elephant. And in the same way, we want to tackle the entire universe. We think ambitiously. We ask big questions and we demand big answers. Yeah, and so whopping of the universe? Is dark matter? This thing that we call dark matter because we don't actually know what it is, right, We only call it dark matter because it's dark because it doesn't interact with light, so you can't see it. And we know it's matter because it feels or it has gravitational effects, and so we call it matter, but we don't actually know what it is. It could be anything. Yeah, it could be a lot of things. There's basically a short list of things it can't be that we've ruled out, but otherwise it could be a lot of different stuff. That's right. It can be bald movement. It cannot be bald move unless you have really really strange bat movements, and I don't know what you've been eating. It's not a crappy universe, No, it's not. But you know, if you hear physicists talk about dark matter, they mostly talk about dark matter in terms of some particle is dark matter, of this particle is dark matter that particle. And that's the kind of thing physicists due because they don't know any better, right, they know the universe that we're familiar with is made of particles, and they sort of just like extrapolate from what we do know into the unknown. And you know, that's how science works. You build from what you know and you explore out into the unknown using the language and the constructs that you have in your mind. Right, I guess that makes sense. Right. You gotta go with what do you know currently, and if that doesn't work, then you know, you have to look for something else. And so we have episodes where we talked about dark matter and also how we know it's there, what it is, and we've talked about some of the things it could be. But in particular, it seems that physicists have sort of narrowed down or at least are mostly leaning on the idea that dark matter is some kind of particle, some kind of particle that we just haven't really seen because it, I guess doesn't interact with electromagnetic forces. That's right. Physicists really like this idea. They like the idea that dark matter might be a particle. It's sort of the way they think. It's like if you ask a carpenter to build a house, he's going to make it for you out of wood, because that's what they're used to. And so physicists are sort of in a rut here thinking about dark matter in terms of particles. And on the podcast a few times we've talked about this, like is that justified? What evidence do we have that it's a particle? What arguments do we have that it might not be a particle? And we sort of glossed over this question. I think what you're saying is that physicists are very particular, they have a certain wave of doing things. But yeah, we've never really sort of dug into this idea of dark matter as a particle, like why do we think it could be a particle or and what do we know about this potential dark matter particle? Yeah, and there was a moment in early dark matter thought where people have this idea, this argument for why dark matter should be a particle, and it goes by the name of the whimp miracle. This is where you que in the angelic sounds, the whim miracle, the dark matter whim miracle. Yeah, and you know, this is really fun because it tells you something about human side of doing physics. You know, it's like somebody was doing calculations and they saw these numbers come together on the paper that really suggested that dark matter should be a particle. Almost like these numbers were a miracle. You know, this this moment when you're doing even theoretical physics, even when you're exploring the possibilities, when things just sort of like click together beautifully and you're like, wait, maybe that's the answer. It works so well that you almost have like a religious experience as a physicist. Yeah, and you wonder like, either this is a coincidence and it's just sort of strange and you know, almost miraculous, or it's the way things really work. And so those moments when you feel like maybe you've cracked something, maybe you've learned something deep about the universe that nobody else is known. I don't know if it's a religious experience, but it's definitely a powerful moment. But it is a pretty interesting concept and it might explain why dark matter is a particle. So to be on the program, we'll be asking the question, and why do physicists think dark matter is a particle? And not all physicists right to use a favorite social media response, a lot of physic this thing that dark matter is a particle. There's a vocal minority out there, and we'll talk about it that are pretty sure it's not. But I like how you phrased it a little earlier. You said the physicists have dark matter thoughts. They have dark thoughts. That seems very brudy and cool. Well, you know, you've got to suffer for your art, right, sometimes you gotta suffer for your physics. Also, you have to have a little existential ank, you know, to get the right idea sometimes, or maybe you need to physics a little to get your suffering. But anyways, as usually, we were wondering how many people out there had heard of this interesting concept of the whim miracle, and so Daniel went out there to ask people on the internet this question. So thanks everyone who game. We played along and answered random questions from an Internet physicist. If you'd like to participate and you're on the verge, please just sit down right to me. Send me an email to questions at Daniel and Jorge dot com. I know you want to think about it for a second. What do you think is the whimp miracle? Here's what people had to say. I have no idea what that is. I have never heard of that before. But the word whimp does maybe make me think of something that isn't strong, So maybe some sort of force that is not very strong. I'm not sure. Well, a whimp is a weekly interactive massive particle, I think, But what a mayor police? I really don't know. Or could it be that if we actually find a whimp, it will explain all our physics problems. That would be a miracle, wouldn't it a jeez? I don't know. Um, the whimp is, if I remember right, weekly weekly interacting, massive particle, weekly interacting. I feel a little more confident about the miracle, though I am. I am not sure. Maybe the miracle is that they can even exist with such weak interactions. I'm really not sure, al right. Not a very miraculous response here on the recognition of a WHIM miracle. No, I think this is something that's sort of buried inside academic physics, and I want to bring it out. I don't see people talking about this a lot online or in popular literature, but I think it's a really interesting and important idea because it really has shaped the way we look for dark matter, the way we think about dark matter, the whole sort of direction that academic physics has taken in exploring this largest of all mysteries. No, I can't believe something called the miracle hasn't caught on with the rest of the population. I guess. And I just realized that we talk about what WHIM stands for. It's an acronym. Yeah, exactly, it's an acronym. We've been confusing people for the last ten minutes, right, so let's explain it. A WHIMP is a weekly interacting massive particle, so w I m P weekly interacting massive particle. And what that means is that it's a particle. Right. It's massive. It's pretty heavy, something like a hundred g v or like a hundred times the mass of a proton, and weekly interacting means that it does not have any very strong interactions weekly there means like feebly interacting, like not very powerfully interacting. Right, it's like a whimpy particle out there, right, Like it's out there, but it's not very strong, doesn't have strong opinions, it doesn't really stand up for itself a whole lot. That's right, And it's sort of the standard in the literature for like the basic theory of dark matter, the vanilla theory, the most common one, the one that sort of fits all the constraints and is the simplest explanation we have. Al Right, well, let's recapt a little bit for people what dark matter is and how we know it's there and why whimp is one of the theories for it. Right, So we know the dark matter is out there because we've seen its effect on the universe. Right, we can tell that there's something out there that's creating gravitational forces that we otherwise cannot explain. When we look at how galaxies rotate, we see that they're rotating really really fast, but there isn't enough gravity to hold them together. Something else, invisible, something that we cannot see, must be in these galaxies keeping them together. And we look back at the history of the universe. We can tell that there was a lot of gravity that shaped the structure, that created like little gravitational wells from matter to forming for galaxies to come together. We wouldn't have galaxies in our universe at this point if we didn't have dark matter sort of tugging on everything and squeezing it together. And everywhere we look we see evidence for this kind of matter that's out there, So we know that it's a matter, We know that it's a source of gravity. We just don't know what it is. Yeah, we sort of know it's there, and we know it has substance to it, right, because it's sort of like its presence is affecting things in very large ways, right, like you said, with the galaxies and with the Big Bang, And it's not just a little bit of stuff out there, it's a lot, right. It's like of the matter in the universe, that's right, it's eighty percent of the matter. So when you look at a galaxy, you're seeing only the bright bits, right, You're not seeing most of the galaxy. Most of it is actually dark matter. And something that throws people off sometimes it's thinking about where the dark matter is. The dark matter is mostly in the same places where the normal matter is because of gravity. Gravity pulls all this stuff together, so it's not like dark matters out there in deep space. There's a big halo of it that surrounds our galaxy. It's sort of like our galaxy is in a big bubble of dark matter. And while there's more dark matter out there in the universe than there is normal matter, it's not as condensed, right, It doesn't stick together as much, so it's sort of spread out evenly through our galaxy. So all normal matter clumps into like stars and planets. Dark matters spread out also between the stars. So in a random cube of space in our Solar system there might be more normal matter than dark matter. But then out there between the stars, this more dark matter the normal matter. It's like some big mysterious blob out there and that we can't see because it's dark and it doesn't reflect or transmit light. But we know it's there. But there are some things we sort of know about it, right, Like we know it's not made out of atoms. That's right. We know it's not made out of our kind of matter. It's not built out of atoms. And the way we know that it is really kind of awesome. It comes from a story from the very beginning of the universe. You know, atoms, of course are built out of quarks, and we know something about how many quarks were made sort of per cubic light year in the very early universe. What like, we know how many quarks were made in the Big Bang. Yeah, we can tell how many quarks were made because the density of corks determines what elements are formed later, Like if you have more quirks and you end up with heavier elements like lithium and helium, and if you have fewer quirks, you end up with more hydrogen and less like lithium and helium. So this is called Big Bang nucleosynthesis. The creation of heavier elements in during the Big Bang is very sensitive to the density of quarks. So we can tell essentially how many quarks there were, and then we can add them up and ask like, well, did we know where they all went? And you know, we don't know where every single exact quirk went, but we can tell that roughly they're all accounted for. And so dark matter is like, you know, a huge open mystery in this accounting book of the universe, So quarks definitely cannot explain it. It's like somebody's going to accounting jail. Let's hope the universe doesn't get audited. Who's gonna do the auditing. Maybe that's the miracle. Don't you know that it's not mad? Of course, because if it was made out of corks, you would be able to see it, like quarks interact with light, right exactly, we know a lot about how this stuff doesn't interact. And quarks have electric charge or either two thirds charged or negative one third chart, which means that they reflect photons and they give off photons and dark matter. You know, people say it's dark, That doesn't mean it's black. Like if you had a cloud of dark matter in front of you, you could see right through it. It's more like invisible matter. And that's because light passes right through it. And as you say, quarks interact with matter. But you know that's not as strong an argument, because you can all will make things that are transparent out of quarks. Right, glass, for example, is made out of quarks, but light passes right through it, and so to say that it doesn't interact with corks is not as strong an argument as to say, look, we know where all the quarks were. There just aren't enough corks to make the dark matter interesting. Like dark matter could be just like dark crystals out there or dark sharts of glass. But you know it can be because you you've accounted for all the courts that were made in the Big Bang precisely. Otherwise you'd have to consider maybe it's some weird combination of quirks, but there just aren't any quarks left in the budget. Once you've made the planets and the stars and the galaxies in the dust, you've used up all your quarks. You know, you have to make dark matter out of something else. Or maybe they were smuggled from another universe. That's my new theory. Somebody smuggled a whole bunch of dark crystals. You're like particle ice, right, show me your papers corks. Well, it's all around this dark matter, and it's pulling our galaxy together, keeping things all nice and cozy, and it's also keeping things pretty cool in the universe. So let's not a little bit more about dark matter and also why we think it could be a particle. But first, let's take a quick break. All right, we're talking about dark matter and the whimp miracle, which is not a food product. It's not like a miracle whimp. I do not recommend you put it on your tast but it is an interesting concept, is whip miracle, which could explain what dark matter is because it's dark matter? Is this big mystery out there and we know a few things about dark matter. We talked about it not being made at Adams, We talked about it being a sort of a key part of the Big Bang. But there are a couple of other things we know about it, right Doniel, Like we know it's not hot. Yeah, exactly. If you put dark matter up on my hot or not on the internet, you're definitely gonna get not because dark matter is quite cold. And some people wonder, you know, like why is and dark matter just neutrinos? Neutrinos are not made out of quarks and they don't interact very much, and we know that there's a lot of them. How do we know dark matter isn't just a bunch of neutrinos, like a huge number of neutrinos. And as much as I like to imagine a universe that's like mostly neutrinos, Also because neutrinos hardly weigh anything, and so if they're like eighty percent of the matter in the universe, then the would just be a ridiculous number of them, you know, like trillions and trillions and quadrillions of them. But we know that it's not just neutrinos. And the reason is that neutrinos have so little masks, so they move really quickly. Most of the neutrinos in the universe are zipping around everywhere. And we think that dark matter is cold, that it's slow moving, and we can tell because the way it clumps and clusters and falls into these gravitational wells that shape galaxies. If dark matter was neutrinos, then they would have spread out a lot more, they wouldn't have tended to form these clumps, and we wouldn't have the same sort of shape to the universe and clumpiness and clusters that we see today. So we're pretty sure dark matter is not neutrinos. It's some sort of slow moving object. Couldn't they be slow moving neutrinos like slow neutrinos. It could be, But what make slow neutrinos? You know, neutrinos are produced in stars and when they come out there pretty fast, and neutrinos I have such a small amount of mass that even very low energy neutrinos are moving near the speed of light. But if these were made at the beginning of the universe, maybe they've had time to slow down. But the beginning the universe was really fast. And if neutrinos hardly interact, then how are they going to slow down? Right? Where's all that energy going to go? Maybe they're just tired, it's been fourteen billion years running around. Maybe they just want to break, Or maybe neutrinos have been building up stamina for fourteen billion years, so they're going to go the distance. They're the underdogs. That's right, Go neutrinos go. But no, we're sure that they're not the dark matter. Dark matter is something else. So you know, we know it's dot atoms, we know it's not neutrinos, but that doesn't actually tell you what it is, right, we know a lot about what dark matter isn't none of this tells you about what dark matter actually is. Yeah, and so if we talked about in another episode about how it could be like some sort of weird new kind of liquid or in weird new kind of sort of matter, but it seems like the big ends out there sort of maybe points to dark matter being a particle. Yeah, there's an argument that was made about twenty years ago that commenced a lot of people that dark matter was probably a particle. And you know, we call it the whip miracle because it's this sort of like funny coincidence of numbers that fell out of equations that all would just work really, really nicely if dark matter was a weekly interacting massive particle, it would just sort of make a lot of sense and tie up a lot of loose ends all the same time. So that's why it was called the whimp miracle, like it would ma rexously come in and explain what dark matter is and all these numbers and evidence that you have, Yeah, exactly. And the argument is actually really interesting because it takes you back to the very beginning of universe when dark matter was made. We know how much dark matter there was in the very very early universe because we see its effect on the shape of the universe. We just did an episode where we talked about the universe ringing with sound and how those waves propagated through the universe. Those waves are affected by the dark matter in the universe. How much dark matter is. They're creating graphity to create the situation for those waves to propagate. So we can tell how much dark matter there was in the very early universe, and we can tell how much dark matter there is today. And there's a lot less today than there was in the very early universe. Most of it is gone. What we used to have more dark matter but then it went away. Yes, we used to have a lot more dark matter than we do today. Like the density of dark matter in the universe now is much lower than it was early on. Well, I guess maybe stepping back first, you said dark matter was made during the Big Bang. That's pretty cool. Like, I guess things were just like mostly energy at the very very beginning, and then it became dark matter. Just like it, you know, corks and other regular matter came to suddenly pop into existence. Dark matter also popped into existence. Yeah, you have this incredible moment where the universe is so hot and so dense that all the fields have so much energy in them that doesn't even really make sense to talk about individual particles. You know, it's like looking at the ocean and asking how many drops there are like drops are not the right way to talk about oceans. So in the very early universe, there was so much energy in all the quantum fields that like particles aren't even really a reasonable thing to talk about. But then as things cooled right and things got more dilute, and as the Big Bang went on, then there was a moment when these particles sort of like became the way to think about the universe, you know, And all this touches on something I think is really interesting, which is, like, what is the story we're telling about the universe? In every case we're like thinking about this in human terms and trying to find the right language to use to describe the universe. None of this is like fundamentally true. If we met aliens, they would be very confused by our description of the universe. And this is how we think about it. You think about the very early universe in terms of like fields filled with energy, and then later on it's reasonable to talk about it in terms of particles. And when that happened, when the transition happened, from like just fields filled with energy, then the energy spread out into all the different fields and made lots of different kinds of particles, corks and electrons and neutrinos, and also dark matter. So before like dark matter and regular matter, we're all just one kind of then it's sort of split into dark matter and regular matter. Yeah, you can think about it that way. And in the very early universe a lot of dark matter was made. And you know, we don't know again what dark matter is or what that field is, but we can measure how much there was because we see the evidence in the cosmic marcrowave background and in the structure of the universe. So where did all that dark matter go? Well, we're not sure. You know, it might have snuck back out into the other universe that I got smuggled in from, for example, re exported. But we think the theory is that dark matter has some way to interact with itself, right, Like, what can dark matter do? We know that it feels gravity, we've never seen it have any other force. We've never seen it interact with electromagnetism, or with the nuclear weak force, or with a strong force. But that doesn't mean there isn't some other force out there that dark matter particles feel some dark force. So imagine this some force where like if two dark matter particles bump into each other, they can annihilate. For example, maybe have a dark matter particle and in its antiparticle and they annihilate and they can turn into a photon or some other particle which can then turn into regular matter. So we have more regular matter than we had at the very beginning of the universe and less dark matter. So we think that some of it converted through this process from dark matter to regular matter. Oh, I see, because if you had more dark matter before, the only way it can sort of transform into something else is through some sort of interaction with itself, right, because it kind of interact with anything else. So it must have like, you know, two random bits of dark matter was crashing each other, turned into energy and then converted into regular matter exactly, And that would be like a portal for converting dark matter into regular matter. There'd be some kind of interaction there that we don't know about yet, some new dark force. So now we're talking about a new kind of matter and a new kind of force in the universe. But you know where else could the dark matter have gone? Somebody needs to restore balance in the force and the balance between light and dark. That's right, This all came out of the writing of young George Lucas. He was a physicist before he was a toymaker. Yeah, the whip Miracle was actually his first name for Star Wars, actually star Whimps. Yes, it fit the logo just as well, but then he thought war would sell more. Yeah, he was gonna call them whimps instead of Jedies, but it didn't work well in focus groups anyway. So this is something you can tell about that you don't just have to say, well, there must be some force. You can figure out how strong that force has to be, like how powerful is that force? How strong is that interaction? Because you know how much stuff there was in the early universe, you know how much of it got converted into stuff now, and you know how much time there was. And you could also play games like, well, you know, below a certain density, dark matter won't find itself anymore, it won't bang into itself to cause these interactions. So you put in a certain number for how strong this interaction is, and it tells you like how the dark matter turns into normal matter, like how many dark matter particles you should have per cubic light year today? But I guess you know here you're assuming that dark matter is a particle. So, like, you know, starting with that, you assume that dark matter is a particle and that these particles interact with themselves. And I think you're saying that, you know, if you sort of do all of the accounting about where all that energy and mass go, then you get a certain number for like the density of dark matter, yeah, the number of dark matter particles left over. And so if, for example, you say, well, this interaction is very very strong, it's very likely to happen, then you end up with less dark matter today. And if you say, oh, well this interaction is very very super duper weak, you end up with more dark matter left today, because that interaction isn't as good at turning dark matter into normal matter. So then you can ask the question, well, what number d you need to get the dark matter we see today, And so you can figure out what that number is, and when you do that calculation, you get a number, and that number is almost exactly the same. Strength is our favorite force, the weak interaction. And so people like, wait a second, that's interesting. Like you know, it's like if I, you know, saw a license plate on the getaway car and then I went over to your house for dinner and you had the same license plate. I'm like, in just in coincidence, Oh, I see. It's like, you know, there's dark matter missing in the universe, so it must be sort of volatile in some way, right, Like it must be. It must attempt to evaporate almost in a way. And if you sort of calculate how volatile it is, you get almost the same volatility kind of of the what we call the weak force, which is one of the four fundamental forces exactly, and we only have a few of these fundamental forces, and the weak force is kind of weird, right, It's really strangely weak. It's not like a you know, a basic simple force that has a strength. We understand. It's a very strange weak force. And so to find out that, in order to explain how much dark matter has disappeared from the universe, you need a force with basically exactly the same strength. That's one of the forces we already have. You're like, that's an interesting clue. So then you thought, all right, we'll assume that dark matter is a particle, and so it needs to interact through some kind of force. And hey, the force it seems to be interacting with is in the same like range as the weak force. So then you thought, maybe this particle that dark matter is is a weakly interacting particle exactly. And then you get to the massive parts. That's only like half of the whip miracle. The other half is the mass. Because this interaction strength the one that tells you, like how much dark matter is disappearing, That just tells you like how many dark matter particles are left over, like per cubic volume of the universe. Now you need to pick a mass right, like how massive is this particle? And if you pick a hundred g v, you know, a hundred times the mass of the proton, which is like how much mass all the weak particles have, the W, the z, the Higgs, Then you get exactly the right mass density for the universe. So the interaction strength gives you the right number density, like how many of these particles there are per cubic volume. And then if you pick the weak scale mass, you get exactly the right mass density. So it's like two miracles that line up perfectly. Interesting. It's like, according to your calculations, a particle that is heavy and interact with the weak force would fit the evidence of dark matter perfectly. Yeah, and you need both of the things to explain it. Like, if we had less dark matter today, then you need a higher cross section or you need a smaller mass. If we had more dark matter in our universe today, you need a lower cross section or a higher mass. So we just happen to have the right amount of dark matter left over in our universe that could be explained by a particle with the weak scale mass interacting at the weak scale interaction strength. Isn't that assuming you know kind of what happened in the Big Bang. Yeah, that's assuming that we know how much dark matter was made in the very early universe. But we're pretty sure about that. That doesn't require you to know anything about whether it's a particle or not. That's just from looking at its gravitational effects on things in their early universe, the cosmic microwy background, these very on acoustic costellations, the whole structure of the universe. So that's an independent measurement how much dark matter there was in the early universe. But this whole thing is a hypothesis, is saying, let's see if it works out if you assume dark matter as a particle, Like can we make this work? You know again, this is not wrecked evidence that it must be a particle. It's like, let's try it out and see if it fits. And then it like clicks in the place perfectly. And then it clicks in the place perfectly over there. You're like, WHOA, that seems compelling. I see you had like this big mystery and suddenly you have clues that tell you like, hey, this kind of particle, a weekly interacting massive particle whimp would fit just the bill for what we see. And that's why you called it a miracle. Yeah, because in two ways it just happened to fit the bill. And so people have this moment they're like wow, they're like got chills, you know, like the universe is talking to us, you know, like they had a religious experience, Like I figured it out. I I have godlike powers to figure this out. Do you think it's a religious experience? Every time you get chills, like if your kids turn on the A C too strongly, you're like, oh, I feel my faith coming back. I definitely do some cursing, for sure, but don't smite anybody. Or but yeah, there is a moment there where you feel like you have an insight or you've seen something clearly which used to be confusing, and so you know, maybe you're a spiritual person or not, but like that feels like maybe you've understood something. I think it's just like when you're cracking a case. You know, when you find a clue and it points exactly the direction that makes sense to you, you find another clue that confirms and and you're like, Okay, this is all coming together. Mm I see. So what you call the whip miracle is more like, hey, it's a miracle we figured it out. Yeah, it's a miracle that these numbers happen to be exactly what you would need for this particle to explain it. And you know, because whimps are a very very common thing in particle theories, like coming from the other direction. We have all these theories about what other particles might be out there, and one of the most common is this thing called supersymmetry, which is this theory that there are all these other particles out there, copies of the particles that we know and love, And in all of those theories they predict a particle just like this, they predict a whimp. A particle around a hundred giga electron volts that interacts about this strong and so that was an exciting moment. All right. So you had this big mystery of dark matter. You were looking for a solution, you were stumped, and then suddenly some people figured out that this idea of a whimp, a weekly interacting massive particle, is sort of like a miracle. It would totally describe what dark matter is. And so let's get into whether or not the whimp is a miracle and whether it does explain dark matter. That's a big mystery in itself. But first let's take a quick break. All right. We're talking about miracle whimp, which is I'm a different part of the grocery store as miracle whip. That's right. It's more than digestive section, right, Digestive aids to supplement for your dark matter so that it's not as as cold, so it moves a little faster. Yeah, heat have your dark matter, get it moving, all right. So we have this apparently miraculous theory about dark matter. It could be a weak the interacting massive particle, meaning like a heavy kind of particle that's slow, that's cool, but that interacts with itself and maybe other things through the weak force, which is one of the fundamental forces. Yeah, and so this became like the standard bearer, you know, all the ideas of what dark matter could be, particles, not particles, weird crazy stuff, dark chocolate floating out in space. This became like the favorite theory. And you know, the way physics works is you have like usually a mainstream idea and then you get the you know, the fringe ideas of other people working on different things that aren't really like the mainstream. But this very rapidly sort of took over the mainstream of physics, and people just were like assuming, all right, this is what dark matter is, or like, we don't know necessarily what that it's this, but this is a good candidate. So it makes sense to invest in like experiments that can look for this and to build complicated theories that include this. And so a lot of work was done because of this wid miracle assuming that dark matter was a whim because it's such a compelling theory, right, like it miraculously fits all the all the evidence we have about dark matter. It did at least it did. Okay, there's a whimp plot to us here, so I guess that was a leading theory, and so people went out there sort of looking for dark matter, assuming that it interacted with the weak force, and so do design experiments kind of along those lines. Yeah, the dominant experiments looking for dark matter as a particle are these big tanks of zenon underground or very very cold semiconductors, and they're all designed to look for a particle that's around a hundred g ev and interacts at the strength of the weak force and would like bump into a nucleus of zenon or nucleus of one of these semiconductors and give you a little signal. So they're like out there trying to catch this wind of dark matter that they thought the Earth was moving through, this wind of whimps. That's a George R. R. Martin book. I think the song of whimps, the song of particles, a song of particles and quarks, the winds of whimper. Yeah. I think we've had episodes where we talk about these dark matter hunt experiments, and yeah, they involved these giant tanks of cool liquid and you kind of hope that the dark matter cloud that the Earth is passing through somehow knocks some of those cold gas atoms around. That's right, And we don't know that dark matter has any interactions at all, you know. The only evidence we have that dark matter has some interactions is this argument that a bunch of it has disappeared since the real universe, and so there must be some interaction there if it's a particle, to convert it into normal matter. And so that was the basis for looking for these things and said, well, let's try to create a big tank of normal matter and hope that dark matter and bumps into it. That uses that same interaction that allows it to annihilate should also allow it to bump into normal matter very very occasionally. Right. It's not something like you're gonna see all the time. It's a very weak interaction. So you need a huge tank of stuff, you need to wait years, and when it bumps into the gas atom or particle, they'll create a little electron that you then sort of measure and you can say, hey, something bumped into my xenon gas. It must be dark matter. Yeah. It's sort of like putting a camera underground for five years and expecting, you know, just black images and then seeing a flash of light and you're like, oh, well, something was here, and we try to shield it from everything else, from muance and from radio active sources in the rock, and they do really careful job, like these are really tour to force experiments to try to make this liquid super quiet, so if you do see something, you're pretty sure it could be dark matter. And so we have episodes where you sort of take a deep dive into these Xenon experiments. But they they've been sort of going around for a while and what have they found since they haven't found anything, unfortunately, So they've been looking for this stuff, and you know, at first they're developed in the technology and then making them bigger and bigger, and they were at the point where they didn't expect to see it like they would have seen it if it happened to interact much more strongly than a whimp, if it happened to have some big surprise, they would have seen it the first experiments. But now these experiments are bigger and they've been running longer, and it's to the point where if the whimp existed and it did have this kind of interaction we're talking about the one that satisfies the whim miracle. We should have seen it already. So these experiments that basically ruled out the kind of particle that would perfectly satisfy the Whip miracle, like you've given up basically, Like people were waiting around to see if they found it and nothing, and so you're hanging up your coat. We're not turning off the lights, and you know, we're not like shut things down and you know, mothballing everything and going home to find new careers as cartoonists or something. I mean, that's bonkers. Yeah, that please don't do that yet. There are plenty of cartoons already. Know. It just means that, you know, the our simplest idea that there was one particle to explain all the dark matter and had a weak interaction, that idea doesn't work anymore. So we can modify the idea. We can try to keep the whip miracle by adjusting it a little bit. You know, we can find reasons why we wouldn't have seen it in these experiments. Maybe it only interacts with other kinds of matter, or we could say maybe dark matter is actually a few different kinds of things and they have like different interaction rates, and it's sort of just like all adds up to give us the explanation we have from the early universe. So people are being a bit more creative now in trying to find ways to keep the whimp and also avoid these experimental constraints. I guess I'm a little confused because I feel like it was only recently where we talked about some of these dark matter experiments. Are you saying that sort of in very recent times people have sort of concluded now that it's not a whimp. Yeah, these results are pretty recent. You know. People have been building up and building up and building up and thinking like, okay, around twenty twenties and when we're gonna have sensitivity to like the whim theory, you know, And now we've seen those results, and it seems pretty clear it's not a whimp. You know, we can keep looking for it. It might be something that interacts more weakly than the weak force. Might be like a very super weakly interacting massive particle that wouldn't satisfy the whim miracle. You need some other way to explain why you didn't get left over with extra dark matter in the universe if that interaction is so weak. But you know, their physicists so they got creative ideas to explain it. You know, none of the ideas are clicked together quite as nicely as the Wi Miracle. But the Wi Miracle didn't turn out to be right, right, Maybe it's light miracle whim left fat all right, So then if it's not a whimp, like if we're pretty sure it's not the whimp we thought it was, the miracle we thought it was, then what are some of these things that it could still be. Yeah, well, there's still lots of other fun theories of dark matter. And there's sort of been two categories. One is other particles, and we talked, for example, about whether dark matter is an axon. An axon is a really weird particles sort of like a heavy photon and interacts very differently than a whimp, and their experiments out there looking for that, so sort of a very different category of possibility for particles. It's also possible that dark matter isn't a particle at all. You know, this entire concept, this whole framing of the question assumes that the rest of the universe is similar to the part of the universe we've been studying for all these years, that regular matter is like a good pattern to start with, but it might not be. Remember, regular matter is unusual in the universe, so extrapolating from it to the rest of the universe seems not so justified. It could be that the rest of the universe, the other matter is something very weird and different. Could not necessarily even be a particle, right, It could be not matters, So it could be just like dark stuff or dark things. Yeah, there are crazy ideas, you know, things called un particles where the particles don't have a specific mass, and this is sort of like weird continuum of stuff where the more you zoom into it, it doesn't resolve into tiny little bits. It just like looks the same no matter how much you zoom into it, which would be really weird and awesome. And then we've talked to the podcast about one of my favorite theories that maybe dark matter is a huge number of tiny black holes left over from the Big Bang. We call them primordial black holes. That theory is under stress a little bit because we've looked for those and we haven't seen them, and there's ways that they can evade our telescopes. But you know, it's sort of less popular theory these days. But the point is we just really don't know. You know, our best theory, our best understanding, our best hypothesis that all clicked together nicely, that seems so promising, turned out to not be the theory of nature. It wasn't a miracle. You debunked America. Physicists once again kill the magic of the universe, prove there are no miracles in this world. That's right, downers since the hundreds. Or like you said, maybe it is a whim, but it's like a modified whim, right like, maybe it's a super a swimp, a super weekly interacting massive particle, or like a weekly interacting super massive particle with some Yeah, all those possibilities is still out there either way. It's all a big mystery and people are still looking for it. I guess kudo is still to those people who built those experiments and look for it, right Like, that's an important way to do progress in science. Absolutely it is. So there's a lot of criticism, you know, people out there say it's ridiculous that we spent tens of millions of dollars on this one idea for dark matter that some people never thought was very convincing. After all, you know, and so you see criticism of this online a lot of like I told you so, Yeah, a lot of I told you those you know, And but you never know with research, right, you'd never know are you going to discover something amazing or are you gonna gonna see nothing? And that's what explorer issue is. That's what research is. Fundamentally, it's exploration. If you knew the answers in advance, it wouldn't be as exciting to make the discoveries. There's no pending on the bag. That's just wagging out the fingers. All right, Well, I guess that's kind of what happens sometimes in signs. Sometimes you come up with an idea that seems to fit perfectly, it seems miraculous. Then you're pretty sure it's going to explain things. But the universe has other ideas. The universe always has a plot twist, The universe always has something out there to surprise us. And it's not like the universe was designed to surprise us. I think that's the issue, that it wasn't designed for us. That we have to adapt our minds to the reality of the universe, and the way we think the universe works is very different from the way it actually does, and so it's a continual readjustment which leads for lots of exciting surprises and plot twists. I guess maybe that is the miracle of the universe. The fact that it's full of surprises and keeps us on our toes and then we can understand any of it at all, frankly, is amazing to me. Or that we can talk about it for an hour twice a week, four hundreds of episodes, that's the real miracle. The fact that you're here listening to us. Yeah, thanks everybody for supporting us on this journey. We hope you enjoyed that. Thanks for joining us, See you next time. Thanks for listening, and remember that Daniel and Jorge explained. The Universe is a production of I Heart Radio or more podcast For my heart Radio, visit the I heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.