Can Dark Matter feel itself

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Speaker 1: Hey, Daniel, what's the best question anyone has ever asked you about dark matter? Well, it's tough to pick. There's so many good ones. But I remember a nine year old who read our book once and emailed me to ask is dark matter bad matter? It must be a Star Wars fan. You know the dark side doesn't sound very friendly. Yeah, dark matter sounds world of unfriendly and mean. It's doesn't never a really good pr team. Well, you know, if there are dark matter physicists out there that we can't see or touch, what do you think they would call us? M I guess they'll probably call us dork matter. We wouldn't seem dark to them and insidious. No, actually we would. I mean if they have their own dark photons, they wouldn't see ours, so we would be dark to them. Man, I can just imagine the conferences. You know, you just spent the whole day saying you're a dark matter. No, you're a dark matter. That's probably actually true. That's exactly what would happen. I am, forhand, a cartoonist and the creator of PhD comments. Hi, I'm Daniel Whitson, I'm a particle physicist, and I'm mostly made out of dork particles. Particles, dark dork particles. That's the worst. Well, mostly in the summer right now. I'm not going outside very much, so pale dark. My dork particles are not very dark. But welcome to our podcast Daniel and Jorge Explain the Universe, a production of I Heart Radio, in which we take you on a mental tour of the universe, zooming down to see what the universe is made out of, what it looks like at its largest scale, what crazy weird stuff we encounter in it, and how we can all understand it because humanity is on a journey, a journey to understand this universe. We started out really kind of clueless about the way things work, and we slowly put together little pieces of the picture. But we're here to give you a tour of what we have figured out and what still remains unknown. Yeah, and it turns out we haven't even left the garage, it seems, Daniel, in our journey of discovery, because a huge percentage of the universe is made out of stuff that we have no idea about. You know, twenty five percent of the universe is made out of something we call dark matter, which is a huge mystery. Right, that's right, And that might seem a bit deflating to you, like, wow, we haven't figured anything out, But to me that's exciting, Like, wow, we haven't figured anything out, which means more discoveries are to come, which means mind blowing revelations about the true nature of the universe are in our future. You know, if you haven't left the garage, that means you can still go back inside. If you forget something that's positive, that's right, Like you forgot to pack your banana. How can you even take a journey of discovery without banana? About these days, who can go anywhere? Yeah, we're talking about dark matter today and it's a huge mystery and it's out there. But a big question is what is it made out of? What is it? Is it a fluid, is it a an energy? Or is it made out of particles or something totally different. That's right, Everything we've ever experienced, everything you've ever eaten or touched or tasted, are made out of quarks and electrons and stuff like that, and so we wonder does the same thing hold for dark matter. But it's a pretty big extrapolation. I mean, the kind of matter that we're made out of that's very familiar, is unusual in the universe, and so it seems kind of odd to extrapolate from this tiny little corner to a huge fraction of the universe. And so particle physicists think maybe it's made of particles, but I like to think, hey, maybe it's made of something totally weird and new. Yeah, we really have no idea. And it's there's five times more dark matter in the universe than there is of the stuff that we are made out of. And it really could be anything like it could be something that's not even a particle. That's right, we've never seen matter that's not made of particles. Everything we've ever experienced can be broken down into these units. But that doesn't mean that everything is like that, Right. We have to be very careful about generalizing. We've seen so many times in physics when we've learned something we thought, oh, this must be the way everything works, and then we discover it's not. Actually and there are bigger stories, is a larger context, and that's the goal of science, is to reveal the actual truth, not just confirm our expectations. Yeah, and so one thing we know about dark matter is that we know that it feels gravity, because that's how we know that it's there. I mean, we can't see it, it's dark, but we know it's there because of gravity. It's keeping the gaxies together, it's pulling things together, and it's you know, affecting the trajectories of galaxies and stars. So we know it feels gravity. We certainly do. It's basically an invention to explain missing gravity. It's like a gravity fudge factor. Yeah. We see things in the universe that gravity has done, and we don't see the source of that gravity. We're like, what's this invisible thing that's making this gravity? And so dark matter is basically an explanation for this gravity that we do observe, and so it has to feel gravity otherwise it doesn't fit the bill at all. But then the question is what else does it do? Does it feel anything else? Does it feel lonely? Does it feel resentful that we call it dark matter? Yeah, we know that it feels gravity, and so the question is does it feel any other forces? And I think more interestingly about this question is does it feel itself like canna bump into itself? Yeah, And we got a really interesting question from a listener. Here's an email I god from Joe from China, who I think really put his finger on the question. He says, if you were to take two particles of dark matter and let them go in a vacuum, the gravity would attract the two particles together. But then would they pass right through each other and keep moving? Eventually would they settle down and be able to share the same point in space like overlapping? Can too dark matter particles? But I guess bump into each other or occupy the same space? Yeah? Do they not into each other and push back? Do they pass right through each other? Can they sit right on top of each other. It's a really fascinating question, and it goes right to the heart of the sort of the science matter of whether dark matter feels itself? This question blow your mind a little bit? Yeah, I did blow my mind a little bit. I just love when somebody asks a new question, a question I haven't heard before, or asks it in a new way. So I haven't really thought about whether dark matter feels itself? Okay, so it's kind of a funny question. Does dark matter feel itself? That means that you know it feels itself. Gravitationally, right, if you have one bit of dark matter here and a bit of dark matter, they're they're gonna be They're going to feel each other. They're gonna be attracted to each other by gravity. But the question is, you know what happens after they feel attracted by gravity. You know, they they're moving towards each other. Are they gonna bump against and and repel it themselves or are they going to pass through each other? Or are they going to stick together? Yeah, it's a great question. I mean, if you and I have gravitational attraction, if we were out in the middle of space, then gravity would very slowly pulls together and we would bump into each other. I wouldn't phase through each other. You woul get uncomfortable really fast. What you're saying more awkward than in space. No one can hear you say, hey, I need more space, Daniel, get off me. But you couldn't write. You'd be stuck together. That's right, we need some other kind of force. But you and I do have other kind of forces, right, the electromagnetic force who keep us from passing through each other. So it's a good way to ask the questions. Yeah, so the question is does dark matter feel itself? And so, as usual, we went out into the world, and in this case, the Internet, to see how many people felt that they knew the answer to this question. That's right, So thank you to everybody who volunteered to answer random Internet physics questions. If you'd like to volunteer right to us at questions at Daniel and Jorge dot com. Yeah, you can ask us questions or get questions from us. Questions in and questions out. It's an eternal flux of questions, right. But think about it for a second. If you were to try to answer the question does dark matter feel itself? What would you answer? And no googling, no googling? What? How about? Wikipedia? Also not allowed? About calling my physicist friend? Totally allowed, totally allowed. All right, Well, here's what people on the Internet had to say. I guess I'm really not sure. I know that some scientists hypothesize the dark matter is comprised of filaments, and if there were these filaments in the dark matter, one would think that they would interact in some way. I really don't think so. I remember it interacting with something visible. That's a good one. The interaction between light matter and dark matter is for obviously still figuring that out, but I would think that it does interact with itself. I don't think that turn dark matter interacts with itself. Um. I think whether discussed two galaxies colliding with each other, but there wasn't an evidence or the details, so not itself. I would guess that dark matter would interact with itself because I know that one guess of what dark matter is is the dark matter particle axion. And it's kind of hard to visualize how a particle can pass through another particle. I don't know. I've never thought about it. Okay, Um, dark matter and dark energy confuse me a little bit. I thought that dark matter was just a theory, and so it's like we've we've seen something out there, and the theory is that this dark matter, but we don't really know. I don't think it can interact with normal matter. I surely if it could pass through itself, we would be able to see it. It might be a dark force, which helps them incorrect. I know it doesn't interact with regular matter. I think we don't know whether how it interacts with itself. It is matter, so it has maths al must interact gravitationally. I don't know. Well, I think it does on some level. I don't know whether there would be quantum or molecular, but I think it's got to interact with itself enough to come together and blobs big enough to cause gravitational lensing that we can see here from Earth. I guess it's interact with yourself, since it's i've gravity. All right, pretty thoughtful answers, you know, people sort of thoughts through it. Yeah, there's some good stuff in here. People are really thinking about what it means to interact and whether gravity counts, and whether gravity would make something bump or not? Does gravity count? Does the Earth feel USh? Yeah? But also some people getting confused about dark energy versus dark matter. That's another rabbit hole there. Does dark energy feel? What is dark energy? Anyway? We have no idea, all right, So the question is that does dark matter feel? And so Daniel Um maybe step us through what we know about dark matter and it's it's not very sensitive. Unfortunately, you know, we wish you would react more. Oh good, that's a good thing, right as a thick skin. Well, perhaps know if you want to get along with your dark matter roommate. But if you want to do experiments to reveal the nature of dark matter, you'd like it to respond sometimes, and dark matter turns out to be very difficult to probe because the only way we've been able to interact with dark matter so far at all is with gravity. Like we said, we've discovered dark matter because we looked at how galaxies spin, and we notice that to spin as fast as they do and hold all the stars in place, you need a lot more gravity than is generated by the stars we see, which means there must be something else they're making that gravity. And we see blobs of dark matter in the sky. We don't see them. We deduce blobs of dark matter in the sky because they're bending the light that passes through them. That's a gravitational effect. So we have very concrete evidence that dark matter feels gravity. That's why we call it matter, not just like dark mysterious thing in the space right because it has gravity. I see. It's sort of like the probervial elephant in the room. You know it's there, everyone knows it's there, but it's, uh, it's nobody's talking about. Sort of. It's also sort of like the opposite of that, because the elf in the room. Everybody is ignoring it, right though you can all see it. In this case, we're all desperately searching for it, but nobody can find it. It's like you're right or right, except you're not. Well. You know, sometimes being exactly the opposite direction is also a way of being right. Yeah, yeah, you know up bean staff yes times minus one. Alright, So we know it feels gravity, but there are other forces that we know about in the universe, and as far as we know, the dark matter doesn't feel them. That's right. And so for example, electromagnetism is a very powerful force and when that we experience every day. The reason that your hand doesn't go through the wall when you're trying to touch it is that the wall has particles that are bound together with electromagnetic forces, and so it does your hand and so you think of them like a chain link fence trying to pass through another chain link fence. That's what prevents you from passing through another object. It's Electromagnetism is also responsible for why things glow right, why we can see things, or why they reflect light. To write like, it's light hits it and then it interacts and it comes back out. That's right. Even blobs of rock that don't glow on their own, we can see them in the sky because they reflect the sun's light, like the moon or like asteroids. And so we know dark matter isn't like that because it doesn't give off light, doesn't reflect light. Light passes right through it. So right through it, yeah, exactly. And I see this all the time in science fiction television, that they encounter dark matter and it obscures their view. Right. Oh, we can't get out of this cloud of dark matter and we can't see where we are. What show are you watching and seen that one? I don't remember the name of it right now. I watched too much science fiction, But you know that, to me, that sounds like a fantastic situation, like, oh my gosh, look we found dark matter. That's right home. We win the Nobel Prize. It's not like, get out of my picture, my photo. It's like I got a photo of dark matter. Exactly. We've been trying to spot dark matter forever and you just figured it out and now it seems like a hassled you. But if you were in a cloud of dark matter, you couldn't tell because it's invisible right. In fact, we think we are in a cloud of dark matter. We think the Earth is surrounded by dark matter, but we can't see it because it doesn't reflect light. It doesn't feel any electromagnetic forces at all. Yeah, we're swimming in dark matter right, Like everyone who's listening to this podcast right now is probably swimming in dark matter. Like right in front of their eyes, is dark matter passing through or sitting there? That's right. Dark matter, we think is spread out through the galaxy. It's fairly dilute, so it's there's a lot of it. There's a lot more dark matter than normal matter, but it's much more spread out we think the normal matter, which tends to cluster into planets and stars and stuff. So we think there's like one squirrel's worth of dark matter in the volume of the Earth. But it's there, it's out there. Wait, what one squirrels worth? One squirrel? Yeah, well it's a standard international unit of mass. One squirrel. Are you not familiar with squirrels? I don't. I didn't usually baked with squirrels. All my favorite recipes are in units of squirrels. You know, a third of a squirrel of oats and you know, half a squirrel of sugar man. That's that's the pandemic for you. It's forcing us into a more primitive Yeah, come over to my house sometime. We'll have a squirrel roast. So wait, the Earth you're saying is in a pool of dark matter that has the same amount of mass as a squirrel. That's what it means. That's right, And there's more dark matter than this normal matter, but the dark matter is more diffuse, it's more spread out, and so out there in empty space is about as much dark matter as there is here in our solar system. You just made me think of a giant squirrel the size of the Earth that a dark matter. You're a very visual person, so I'd love to see a drawing of that of a squirrel attacking the Earth and earth size squirrel attacking us. Finally, the aliens have come and they're just giants space squirrels. You did it again. Just because it's a dark matter squirrel, you assume it was bad, Daniel. Yeah, maybe it's a friendly planet sized space squirrel. Yeah, you're right. Now, so we we know that it doesn't feel electromagnetism, and that's principally because it's dark, we can't see it. But there are other forces out there, right, we talk about the weak nuclear force. That's a very very non powerful force. It's a weak force between particles. Neutrinos feel this force, Leptons feel this force. All the particles we know feel this force, but it's not very powerful. Like neutrinos pass through a light year of lead and have a fifty percent chance of feeling anything using the weak force. Our particles also feeling, like the particles in my body that they feel the weak force. Yeah, all the particles in your body, on the electrons and the corks, they all feel this force. It's responsible, for example, for a beta decay. So one of the corks in a neutron can turn into another kind of quirk which turns your neutron into a proton. That happens through the weak force, and that's why a little neutrinos emitted along with the electron that's created. So all the particles in your body are sensitive to the weak force. Neutrinos that pass through you can interact with you. It's just very very un likely, like there's a hundred billion newtrinos passing through your fingernail every second, and you don't feel them well, and we know that the dark matter doesn't feel it. We suspect that dark matter doesn't feel it because we've looked very very carefully. We've set up these big underground experiments filled with very quiet, heavy liquids, and we have a lot of those atoms in there, and we hope that a dark matter particle would come by and bump into one of those xenon atoms, for example, and we could spot it. And this is similar to how newtrinos were first discovered with vast tanks of underground liquid waiting for a little recoil from one of those atoms. So we have a similar in spirit kind of experiment done underground to look for passing dark matter particles. All right, So, and there's also the strong force, right which that one is pretty important, but dark matter doesn't feel that one either. That's right. The strong force is very powerful and so unlike the weak force, which you know, if dark matter felt it, it would be difficult but possible to detect in large underground experiments. If dark matter felt the strong force, we would notice that right way. I mean, it's very powerful. It wouldn't take a very big experiment. If dark matter felt the strong force, then it would react to almost every particle in your body and every particle in the Earth. It wouldn't be hard to notice. It would break up particles all the time. It would be a very powerful force. All right. So then those are the four main forces that we know about, and as far as we know, dark matter only feels gravity. And so the question is what what happens to dark matter? Then? Does it bump into itself or does it ignore it even itself? And so let's get into the question of does dark matter feel itself? But first let's take a quick break right there. We're talking about dark matter and whether it can feel itself, and so we know it feels gravity, and we know it doesn't feel the other forces. So I guess the question is, you know what happens when too dark matter? Part of goals if they are particles get attracted to each other, what's going to happen and are they gonna bump into each other or just totally ignore each other. It's a really fun sort of thought experiment. This is exactly what Joe is asking. And so you know, let's assume for the dark matter only feels gravity, that no other forces we don't know about that are involved, that it's just a gravitational thing. And you know, that is totally possible. It's possible for a particle to have mass and no other interactions. It's sort of incredible, but it would be something that we've never seen before. But hey, this physics is full of surprises. Well, because we know about neutrinos, right, Neutrinos are also sort of selective about which forces it feels. Yeah, we don't understand that at all, Like why do neutrinos only feel the weak force? You know, why don't they feel the strong force? Why don't electrons feel the strong force. It's just sort of like a thing we observe. We notice these things. All the corks feel the strong force. None of the electrons or neutrinos or muans or towels, those particles feel it. Why not? I mean, we don't know. We know that neutrinos don't feel elect a magnetism because they don't have a charge. Only things with electric charge can feel that. But we don't understand this. It's just it's something we observe, something that we hope one day to explain From the bottoms of principle. But at this point we're pretty clueless. All right, So then what happens, Like Joe put it, What if I take two particles of dark matter in an empty universe, said I'm a meter apart, and then I let go. Well, they attract each other, right, gravity pulls them together. But then now imagine what happens when sort of their edges meet, right, what happens that one blob versus the other blob. Now, if this was normal matter, when their edges meet, then they would repel each other. They would bump, literally, because they have forces to push against each other with. But if all you have is gravity, gravity can't push. Gravity only pulls. Right, you have two objects with positive mass, they only pull on each other unless you have negative mass particles. And we have a whole other podcast episode about that. Gravity can only attract. So there's no way for gravity to put ish, which means these particles just get closer and closer. There's no repelling force, and eventually they pass through each other. Right, Because there I guess point particles right, Well, we don't know if they're made of particles, right, but imagine they are mative particles. So I think these arguments hold even if they're made of some weird other kind of thing that you know isn't divided up into little particles. But they can basically cohabitate the same space. Right. The only thing that prevents things from overlapping are the forces. Right. The reason that one proton doesn't want to be next to another proton isn't because the stuff in them is like you're crowding me or something. It's all about the forces. As you said, if you imagined that their point particles, they have zero volume. So the only thing that gives distance to things are forces that repel, and gravity can't do that. Well. I think what's interesting to think about is that you know they wouldn't share the same space for very long, right, because if you set them a meter apart, they're gonna ask some velocity by the time that you get to the center, and so they'll just keep going. Yeah, they pass through each other, but then gravity will slow them down, stop them, turn them around, and they'll come through each other again. So like slash back and forth forever and right forever, right, because there's nothing, nothing would be making you lose energy. Yeah, in your hypothetical universe where there is nothing else, then they would slash back and forth forever. If they had some other way to lose energy. There's like larger blobs of dark matter that are sort of you know, equilibrating them or something. Then eventually they would slow down and then stop and sit right in the same place. But maybe not dark matter what you're saying. In in the real universe, that would it is possible out there that dark matter has settled. Yeah, it is possible. In fact, we think that dark matter is not moving very much. There's two theories of dark matter. There's warm dark matter and cold dark matter. And warm dark matter just means that the dark matter sort of zipping around everywhere. It's like has high velocity, and cold dark matter means it's mostly sitting around. It's like sluggist chilling and not moving very fast. It's chilling matter exactly, chill dark matter. That's for a statement of its feelings. Man. Yeah, so we think the dark matter is cold. It's cold dark matter, and so it's not moving very much. But yeah, if you imagine so, like a gravitational well constructed by something else, some larger object than two pieces of dark matter in that well would fall together to the bottom of it, and then they could just sit on top of each other. And I guess the same would happen with dark matter and regular matter, right, they would technically, like we only interact with gravity with dark matter, and so therefore we would just pass through us back and forth. Oh that's a good point. Yeah, And I think you're right, And I think dark matter is passing through us right now, or we're passing through dark matter in the same way that like neutrinos passed through us. But if there literally is no interaction other than gravity, then you're right. Dark matter can be in you and you can be in dark matter. Oh man, I feel like now we're getting ethereal here about dark matter and spiritual It's in you, in all of us, and it's not just a hypothetical question. We sort of did this experiment on a massive cause, a logical scale. Oh yeah, we did it. Oh we didn't sort of do what I mean, as much as astronomers do anything. They just sort of watched the universe do stuff. You say, astronomers don't do anything, Well, they don't construct experiments, right, They don't go like, let's build a sun that has these properties and distribute these planets around it and let it go. Right. They don't do that except in simulations, but they just look around and find that situation because the universe is so vast and so varied that they can usually find whatever they're looking for. The shoppers, not doers. I love astronomers. Nothing I say should be construed as negative towards astronomy or astronomers. But yeah, they're mostly observers, right, And what they found was these two clusters of galaxies that had slammed into each other. Yeah, it's called the Bullet cluster. Right, it's called the Bullet cluster. And this is a collision that happened a long long time ago. And by galaxy cluster, we just mean literally a group of galaxies, because galaxies orbit each other and are sort of grouped to either the same way that like stars are grouped together into a galaxy, galaxies themselves are grouped together into clusters of galaxies, and sometimes these things collide. Right. Dark energy is pushing the whole universe apart, creating spaces between things, but there are still forces and still directions that can pull things together. And so gravity pulled these two clusters together and they slammed into each other, and so we basically got to see this experiment happened. Yeah, you can look it up online if you google bullet cluster, and they're pretty spectacular images. Like, imagine a cluster of galaxies slamming into another cluster of galaxies. It's like a special effects bonanza. It's like a large hadron collider on the biggest scale possible, the largest, the large gaxy collider collider. Yeah, and of course you can't see it happen in real time. The whole collision took, you know, hundreds of millions of years. What we're looking at when we see it in the sky, it's just the after effects, and we can reconstruct what we think happened from what we're seeing now. And so, you know, the clusters are made of sort of three basic constituents. You have the stars, of course, so the things that are visible in the galaxy, and then between the stars you have gas, the things that make the stars. And then of course each one has dark matter, these huge blobs of dark matter. Remember that most of the things that are out there in the sky, the visible galaxies, have these halos of dark matter around them. So wherever you see stuff in the sky is probably where the dark matter is. They tend to follow each other. We've never seen or have we seen dark matter by itself. We have noticed some clumps, like that's what gravitational lensing is, so it is distributed. It goes out further past the edge of the galaxy, Like each galaxy has a halo that's longer and wider and larger than the galaxy itself. And we have seen blobs of dark matter in the universe by themselves. They do this gravitational lensing, but for the most part, the structure they follow each other. And that's not an accident. That's because of gravity. The structure of dark matter created the structure of normal matter, like created these gravitational wells for normal matter to fall into. That's why we have galaxies and stars and planets and squirrels. It's like, which came first, the dark matter or the galaxies, And the answer is the dark matter. Yeah, well, all the matter was created, i think at the same time, but dark matter there's more of it, and so it has a huge influence on the shape of the universe. So we saw this collision happen and we looked at it, and what we noticed is that when these two galaxy clusters passed through each other, the gas reacted like you would expect what to happen when two huge piles of stuff hit each other, right, big special effects, explosions, lots of friction and radiation at the edges and all that kind of stuff. And so you could see the gas not just kind of mixing with itself but actually like violently reacting. Oh yeah, these things are moving really fast. Absolutely, like if you shoot gas really fast and another cloud of gas, then things would happen. Things will happen, and they slow down, right, And so that energy had to go somewhere, and that energy went to heating up the gas and then it radiates, and you have all this friction between these two colliding clouds of gas. But the dark matter in these clusters got stripped way. It passed right through. It went right through the clouds of gas and right through the clouds of dark matter from the other galaxy and passed through to the other side like it went through the windshield or you know, thinking maybe like in the matrix, remember when they're fighting those ghosts and they're in the car and they slammed the brakes and the ghosts just keep going. Yeah, kind of like that, or like you're walking down the sidewalk with your kid and they notice something interesting and they stop and you just on your phone, and so you don't even pay attention to a block later, you're like, wait a second, where's my kid? I mean, that's never happened to me. They're reacting with some gas or is it running to another kid that you didn't spot? Right, there's the analogy, and the two parents just keep going looking at their phone. We're really modeling responsible parenting. I know, we're really talking about our feelings. And so we can see the dark matter came at the other side because we use gravitational lensing, we can see it distorting the light from background galaxies. So dark matter passed right through normal matter, gas, etcetera. Collided and so they got separated. So we really can't see in that case, this is a you know, collidab lab of dark matter with another lav of dark matter. What happens, Well, they massed right through nothing exactly, nothing happens. It just it just passes right through at least exciting collision. Ever, Well, I'm sure if it's exciting for them to lose their kids A few blocks back. All right. So we know then for sure through observational experiments that dark matter doesn't seem to feel itself, at least not in the way that we know about or with any forces that we know about. And so the question is, is there maybe another way that dark matter can feel itself that we don't know about? And so let's get into that. But first let's take a quick break. All right, Daniel, have you have you find your child yet? I've got your child and still looking for mine. Oh well no, wait, this one looks like, oh man, what take mishap here? Yeah, it's like parent trapped galaxy style. All right. So we're talking about dark matter and whether or not it can feel itself. And we know you can feel itself with gravity, but gravity doesn't help you really, you know, permanently feel or clump or things like that. And so the question is it could there be maybe another way that dark matter interacts with itself that we don't know? There absolutely could be, I mean, the existence of dark matter and the existence of dark energy, and our sheer ignorance about how the universe works, how it begins, and what controls its largest dynamics tells us that there's a lot to learn about the universe, and so there's certainly could be a whole other force out there, some force we've never discovered, some force that only dark matter feels and feels with other dark matter, and that might be the reason we've never seen it, the dark side of the force. Yeah, there's nothing preventing another kind of force from existing. It's not like we have any idea as to the number of forces that there could be, or is any limit to how many forces there are. It's just sort of like a list of stuff we've seen so far in our attempt to understand it. And so that's it. That list is almost certainly incomplete, the way our list of particles is incomplete, or the list of Beatles is incomplete. Right, Like every year he's scovered like a a thousand new kinds of Beatle. It's amazing. Yeah, Ringo, Paul Bob Robert And that was such a bad joke. I'm sorry. I did learn about a new Beatle last week. Actually, I discovered that they brought Ringo in, that they fired the original drummer, and he's still kind of better about the fifth One of the fifth Beatles is usually refers to their producer. But yeah, that the Ringo was not the original drummer. Yeah, not a music podcast, So let's stick to our strengths here matters. Yeah, well, this is a really interesting idea. I think that the you know, because it's it's like totally possible for there to be a whole another universe on top of us, right, Like there could be a whole set of other particles to feel, a whole other set of forces, but we just can't see them or feeling or touch them. Yeah, there's a whole party going on, and we weren't invited, and even if we had been invited, we couldn't even really attend because it just ignores us completely, right, And we know that there's a lot going out there that we can't see. Right, what we see in the universe is a tiny fraction what's actually happening, even just here on Earth with your eyeballs. Your eyeballs see a little sliver of all the electromagnetic radiation that's out there. There's a lot of stuff going on you can't see. And imagine if you could see, you know, cell phone signals and television signals and radio waves, the world would be a much different looking place. And so we see a little fraction of what's actually happening. Wow, and good thing to write like it be overwhelmed me. I mean, you can get Netflix in the Hulu for free, but you know, cloud based delivery would be a different kind of thing. All right, So there could be another force out there, and it's im possible the dark matter feels it then when we see it in these collisions, that was interacting with itself in another way. Yeah. So there's sort of two different categories of ideas. One is, could there be another force that dark matter feels and uses to interact with our kind of matter, some new kind of force we've never seen before that mediates the interaction between those two kinds of particles and that we're looking for and we haven't ever seen, and we're looking for that in the same way we're looking for like its interaction via the weak force. We have these big tanks of underground normal matter particles and we wait for dark matter to bump into It doesn't matter if it's the weak force or some other new kind of force. Like would it be really hard to, for example, to take one squirrel that's flying through the earth. Yes, And now you're talking about one squirrel particle and it's flying through like a ton of liquid zenon. These are very difficult experiments, and they're done underground, like a kilometer underground to keep them cold and isolated from all of the sources of background. They're very hard experiments, and they haven't seen anything so far. Right, But can you say it conclusively, like there's a pink squirrel out there with the purple polka dots, but we haven't found it on Earth, so therefore it must not exist. You're right. We can't say it does not exist. We can say it's less common than one squirrel per cubic volume per year. Right, We can just say how likely it is to exist at a certain level, and so it could still be there. And people are still running these experiments, and the longer we run them, the more we can push those limits. We can say, if this force does exist between standard model A matter and dark matter, that it must be even weaker than this limit, than this limit, than this lem We keep pushing that down. But there's a whole other category, which is like maybe dark matter just feel stuff with itself, not with our experiments, not with our bodies, but just between itself, like is it possible that dark matter bumps into itself and feels something right or repels each other with because you're telling me that this idea that it only feels gravity is sort of on thin nice like maybe there's evidence that maybe it does feel something else. Yeah, And this is really hard to probe because you can't do experiments using normal matter material to probe dark matters interactions with itself. You need to like build your experiment out of dark matter, right, So all we can do is look at how the dark matter is laid out in the universe, like where did it end up, and then try to see is that consistent with how it should end up if it only feels gravity, because if it is bumping into itself, that will change the way it's laid out, just like if it's hot or cold, if it's fast or slow, will also change the distribution of dark matter. So we look at the distribution and try to ask is it consistent with gravity only or do we need another piece of the puzzle aid And what do we see do we see out there in the universe that maybe it does feel another force? We see that mostly gets things right sort of on the scale of galaxies, and larger like the distribution of galaxies and how they cluster together and superclusters. That all seems right, and you run a simulation of the universe with dark matter and normal matter and dark matter is only gravity, it mostly gets that right. But then if you zoom in, like on the smaller scale here, smaller scales like less than a galaxy siza, then sub galaxy, then it's not quite right. It doesn't predict things the way we see them. For example, we know that we have a big blob of dark matter in our galaxy, and we know it's the densest at the center just because of gravity, But the exact shape of that blob is not something that we understand, like dark matter with only gravity predicts it sort of like peaked right at the center, that the center is like a hot spot for dark matter. But we go out and measure it, we see it's much more spread out than that. It's like much more of a rod core, not so much like peaked like a cusp, and so that's not something we understand. Fuzzier than what you would expect. You would expect it to sort of cluster more, but it's actually kind of fuzzy it's like a big fuzzy squirrel, not a little tiny intense squirrel. Yeah, it's a hedgehog or or something like that rather than a tiny little squirrel. And if you change your simulations and you add a little bit of bumping this like a little bit like maybe sometimes dark matter does bump into itself, that describes what we see better, meaning like a little bit of a repulsive force with itself. You just add in a made up force. You're like, we don't know, but let's see if it describes a situation, just like we made up dark matter to describe this gravity we were seeing. We're like, all right, well, now let's give another force to these particles that are creating the gravity, so that the distribution of gravity is the way we are seeing it. Let's make something up about something that we've made up. You're talking about all the physics, now, man, we'll come to science. What if that thing we made up for the thing we made up, that's something else we make up. Yeah, and so this is sort of the state of the art. Now we're like looking really carefully trying to understand where is the dark matter? Does it agree with what we predict and it turns out it doesn't quite agree, and there are other little details that the gravity only theory is getting wrong, so that they're sort of strong maybe or at least a hint of an indication that maybe dark matter does feel a whole new force or could it be one of the old force. It couldn't be one of the old forces. It would have to be some sort of self interaction. So this theory is called self interacting dark matter, and it solves some of these nagging problems that we've had in the gravity only theory of dark matter. So that makes it compelling. It's not proof at all that this is happening, but it's like, huh, we've had this problem. We add this new idea. It gives a better explanation. That's cool. And you always have to be careful when you're adding more stuff to your theory. Right, you'd like the simplest possible explanation, but if that doesn't work, then you have to add a little widget to your theory, like maybe what if there are two kinds of squirrels matter? Yeah, and and that's a whole other idea, right, Like if dark matter made of particles, is it one kind of particles? Is it two or is it ten? Didn't they decay from one kind particle to the other? Do only some of them have self interactions? I mean, look at the particles that make up our kind of matter. There's all sorts of complicated interactions with weird rules we don't understand. Why would we imagine dark matter would be like simpler. It could be different shades of dark matter, the darkest matters squirrels and cats. And and then then you wonder, like, all right, well, maybe dark matter could feel itself and that would explain like the weird distribution of gravity that we're seeing. But what about the bullet cluster? Right? Didn't we do this experiment because the two clouds of dark matter didn't repel each other. They didn't repel each other. But people went back and looked at that again, and they said, all right, well, can we explain this and have some amount of interaction in there, because it's not easy to tell exactly where all the dark matter is very precisely, like maybe some of it did interact and got sort of stuck in the middle. Most of it didn't, but maybe some of it did. You know. It's like we made it up in the first place when we just you know, it's like there's a first pass and there's a more precise test, right, you know, first you try to understand the broad strokes. Then you really dig in and see if it gets the details right. And the other thing is it turns out that this is not a great dark matter collider. It's too big dark matter. Well, dark matter is very dilute, and so if you throw a huge, very dilute cloud of dark matter at another one, most of it's just going to pass through anyway. Like, dark matter is not as dense as these gas clouds. It's sort of like like the stars. Like if you have two galaxies that pass through each other, mostly the stars don't collide because the stars are embedded in huge, vast empty spaces, and so they don't really collide with each other head to head. So you were saying, like, if I take two dark matter squirrels and I throw them at each other, they might repel one against the other. But if I take a whole cloud of squirrels that are really far apart, they might just pass through each other like we've seen in the cluster. Yeah, and they would mostly miss, just like the large Hadron collider what we do is we collide to clouds of protons against each other, because one proton will never hit another proton, they're too small. So we have like ten to the ten protons and one cloud and ten to the ten to the other, and we get you know, ten or twenty or fifty collisions. And so it's not that easy to make things collide. And so it turns out the bullet cluster is actually also consistent with dark matter feeling itself at sort of a low level of tweak how you look at it. Yeah, and so you know, the game here is come up with a theory um model that explains everything that we do see all the same time. And so you have to add some of this stuff to explain the distribution of dark matter and the core versus cup effect to the center of the galaxy, but not too much that you mess up the description of the bullet cluster. So you sort of hemmed in on both sides there. Interesting, But I guess maybe the takeaways that the answer is, yes, maybe dark matter does feel itself. It's totally possible, and there might be some indication out there that it does. Yeah, it's a new idea and it's gaining traction in the community self interacting dark matter. So it's possible that yes, dark matter could feel itself and there could be multiple forces that it feels all right, So maybe dark matter does have feelings, Daniel, It might have a lot more feelings than we could ever understand. Shades and shades of nuance. Well, I think it's all again, just all of this is just an interesting example of how there are things in the universe happening that we don't really know about or see or feel or effective. We're only seeing a tiny fraction of this grand play of the universe, and we're making huge, broad conclusions about how the universe works from this tiny little corner of the stage. And there's a lot going on that we haven't ever observed. And at least we know now. We know that we're ignorant, so we know to be humble and we know not to over generalize our ideas. But it also is tantalizing because you hope that one day we will see the rest of the action, We will get a glimpse for what's really happening. We might all be swimming in a giant squirrel up mr. I like to think of myself as a physics raccoon, because it's sneaking late at night to steal other people's garbage with dark circles on your eyes. That makes sense eating garbage. All right, Well that's pretty interesting and cool and or chill um, and so we hope you enjoyed that discussion. Thanks Joe for writing with your question. And if you have a question you'd like us to explore on the podcast, please write to us. We love your questions. Two questions at Daniel and Jorge dot com. A bit mysterious. See you next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of I Heart Radio. For more podcast from my Heart Radio, visit the I Heart Radio Apple Podcasts, or wherever you listen to your favorite shows.