Today, Explained - Unexplainable

Episode Date: November 24, 2020

Scientists all over the world are searching for dark matter: an invisible, untouchable substance that holds our universe together. But they haven’t found it. Are they chasing a ghost? We want to hear your feedback on Unexplainable! Email us at [e-mail] or let us know on Twitter @today_explained Transcript at vox.com/todayexplained. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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And it's our last before we take a little break for American Thanksgiving. We'll be back on Monday. So we wanted to go out on a high note. Today Explained producer, reporter, sports correspondent, musical director Noam Hassenfeld has been hard at work on a new show with Vox's science team. So we thought we'd bring it to you today. It's called Unexplainable. It's a show about the biggest, strangest questions in science that still haven't been answered. And we want to hear what you think of it, so tweet at me or at the show.

I'm at Ramis Varum. The show is at today underscore explained, or you can send us an email. We're todayexplainedatvox.com. Here is Noam. I remember the first time I saw the movie Men in Black. I must have been in like third or fourth grade, and the whole thing was pretty fun. It was breezy, action-packed. And then I got to the final scene. The camera zooms out. You see trees all in New York City, the Earth, the Moon, Saturn, entire galaxies until it zooms out so far that you can see that everything is just inside a tiny marble and there's a whole bunch of other marbles in a bag and these aliens are just tossing them around. It kind of terrified me. I had these nightmares about being stuck in that bag of marbles,

just thinking that everything I knew and everyone I loved was so small. Even our universe was insignificant. But that was just a movie. This is a story about the scientist who discovered it was real. Not the alien's part, to be clear, but she discovered that everything we can see, everything we can touch, everything we know, is just a tiny sliver of what the universe really is. So it's the late 1960s. It's evening time. The stars are coming out.

Just outside of Tucson, Arizona, out in the desert at what's called Kitt Peak National Observatory. Science writer Ashley Yeager. Vera Rubin, an astronomer who's very interested in how galaxies work, and Kent Ford, her collaborator, are getting ready to point one of the telescopes at Kip Peak to the night sky. They've got this new, cutting-edge telescope, and they're pointing it towards stars at the edge of a spiral galaxy called Andromeda. And they're trying to get the speeds of these stars. How fast are these stars going around Andromeda? Up until that point, there was an assumption about how stars in other galaxies would move. That the stars closer in would fly around the center of the galaxy,

and the stars farther out would go slower than the stars closer in. The idea was that they would basically work like planets in our solar system. So Mercury is flying around the sun because it's so close in. It's going super fast, more than 100,000 miles per hour. You know, it's getting all of that gravity from the sun. Pluto's super far out. Getting less gravity from the sun, going like 10 times slower. So it's kind of just like, do-do-do-do-do, I'll get there someday.

But this idea that stars further out would go slower like Pluto, it was just an assumption. Almost no one had attempted to do things far out in a galaxy. Vera passed away in 2016, but we've got recordings of her old interviews. And I was after something big. So they're out there at the telescope, slowly gathering data, and as they do, Vera notices something unexpected. We found that the stars very far out were going almost exactly as fast as the stars in the interior. These stars, these hot young stars in Andromeda, are moving way too fast than what Newton's gravity would allow for.

They're going so fast that you'd expect them to just fly off, slingshotting into space. I mean, I don't know if I'm allowed to swear, but I think it was kind of like a what the fuck moment. You know, like, it was just so different than what everyone had expected. There were two equally unsettling explanations. Either Newton's law of gravity is wrong, or we have no clue what's going on at the outer edges of the galaxy. There's got to be something happening all the things we don't know. I'm Noam Hassenfeld.

This isn't a show about answers. It's about the questions. Why they matter, what's standing in the way of a solution, and how to grapple with the unknown. We're starting with one of the biggest scientific unknowns. What is the universe made of? That observation Vera had out there in the Arizona desert, it set her on a path to confronting this massive scientific question. And it would ultimately upend

what physicists thought they knew about the universe. But it's not like Vera had a eureka moment on the spot. I mean, in retrospect, I was terribly stupid because I didn't get excited about it. When she made her observation, Vera had a few options. Option one, she could dismiss it, just like say it's not a big deal. I mean, when you first see it, I think you're afraid of being, you're afraid of making a dumb mistake. You know, that there's just some simple explanation.

Option two, she could do something that happens often in science, come out with a grand sweeping conclusion based on limited data. And of course, you know, in science, it can be a bit of a race. But Ashley Yeager, the science writer, says Vera went for option three. She wanted to collect more data. She never assumed anything. She was always like, OK, well, I don't want to just believe that. What's the data to support that evidence? Honestly, this is one of the things that makes me admire Vera so much. She had this chance to wow the scientific world with some bonkers conclusions, and she waited. She was careful.

And so Vera and a couple other people, they really start to do a systematic study of galaxies. And it wasn't just a one-off in Andromeda. They all show this bizarre behavior of stars, these stars out far in the galaxy moving way, way too fast. The data pointed towards an enormous problem. The stars couldn't just be moving that fast on their own. They needed some kind of extra gravity out there acting like an engine.

And there had to be a source for all that extra gravity, which might mean... There's a shit ton of matter out at the edge of the galaxies that is tugging those stars along. Except even when scientists looked through the most advanced telescopes, no one could see any of it. That raises the next question of, like, what is it? Vera and other astronomers could only guess.

Maybe it's black holes and, you know, really faint stars and planets that we can't see. They would say that it was baseballs they were teasing, obviously. But as they're looking out there, they just can't seem to find any kind of evidence that it's some normal type of matter. At a certain point, they basically have to say, We really have no clue what it is. And this is where it all gets way weirder. There's this invisible mass, what astronomers call dark matter. Dark matter. Hanging out at the edge of these galaxies,

tugging those stars along, making them move super, super fast. Way faster than we would ever have assumed. To move this many stars, there would need to be a staggering amount of dark matter. More than all the rest of the normal matter in the universe combined. But it was completely invisible, which naturally raised a lot of questions. What is this stuff? No idea.

Is it the stuff that makes up you and me? Is it normal matter? What does that even mean? Which naturally raised a lot of questions. What is this stuff? No idea. Is it the stuff that makes up you and me? Is it normal matter? What does that even mean? Or is it something completely different that we have to kind of rethink our entire structure of the universe? That's what it was starting to look like. Which is probably why in the 60s, the idea wasn't exactly catching on. Decades before Vera, other scientists had seen stars and galaxies moving too fast, and they'd had the same three options Vera had out there in the desert. Some chose option one. They dismissed it. Some went for option two. They made sweeping

conclusions about dark matter, but they didn't have all the data that Vera had, so the idea of dark matter just kind of floated at the fringes of science for decades. I mean, it takes a lot to make scientists rethink the entire structure of the universe, especially when it involves... A shit ton of matter. ...that we don't understand. I think many people initially wished that you didn't need dark matter. It was not a concept that people embraced enthusiastically.

And then came Vera and all of Vera's data to really turn things around for dark matter. You know, she does 20 galaxies and then 40 and then 60. And they all show this bizarre behavior of stars out far in the galaxy moving way, way too fast. I mean, it just piled up too fast. Observations were undeniable enough so that most people just unenthusiastically adopted. So at that point, you know, the astronomy community is like, okay, we have to deal with this.

Vera convinced the world where previous scientists couldn't. And I think it's because she did this in so many galaxies. You know, we're talking 60 galaxies. There was really no denying it. Ultimately, in a 1985 talk to the largest body of astronomers in the world,

almost two decades after that moment in the Arizona desert, Vera had enough data to declare her grand sweeping conclusion. Nature has played a trick on astronomers who thought they were studying the universe. And she says that we have been studying matter that makes up only a small fraction of the universe. The rest of the universe is stuff that we don't understand and we can't see yet. What makes up you and me and the planets and the center of the galaxy, like, that's normal matter, that's everyday matter, but that is not the bulk of the matter in the universe. I mean, you're totally flipping the script of what we understand.

It's kind of the equivalent of the realization that the Earth isn't at the center of the universe. Now, not only is Earth not the center of the universe, now the matter that we knew is not the center of the universe. Now, the matter that we knew is not the center of the universe. Like, that most of the matter in the universe is some crazy thing that we can't even describe. We have no idea what it is.

It's, I mean, it's mind-blowing. It kind of hurts your brain to think about it. It's almost an anti-Eureka. Vera Rubin didn't discover something. She discovered how much we don't know. A blank in our knowledge. Today, that blank is still there. And as they try to fill it, scientists are gazing off into deep space,

gathering clues at the bottom of old mines, and wondering, what if none of this is real? That's After the Blank. Support for Today Explained comes from Aura. Aura believes that sharing pictures is a great way to keep up with family, and Aura says it's never been easier thanks to their digital picture frames.

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Brian, you want to introduce yourself? Yeah, I'm Brian Resnick. I'm a science reporter here at Vox. Okay, Brian, I get that, like, there's a ton of dark matter out there. I mean, more than all the other matter in the universe combined. But aside from what it does, like speeding up stars and galaxies, what is the actual stuff of it? What is dark matter made of?

Yeah, so no one knows. So if you don't know, if you've been confused, you're where science is at. We don't know what dark matter is. Do we have any kind of guesses about what kind of thing it is? Well, scientists think it's a particle. What's that? Is that like an atom? Smaller than an atom? Smaller than an atom. So particles are the basic building blocks of nature. They're like the smallest Lego brick that makes up reality. And so we think that dark matter just might be another one of these little Lego bricks. Okay. But it's like, as a building block of nature, it's really, really weird. You couldn't touch dark matter if you tried. Like it would just go right through your body. It's kind of like a ghost. Okay, so it's invisible. You can't touch it.

It kind of sounds made up. It's not made up. This is what our observations lead us to think. Astronomers have even seen these galaxy clusters smash together. And when this happened, the dark matter just went straight through. Huh. And that's what makes dark matter like really hard to find. Yeah. I mean, how do you find something that is invisible and untouchable? It kind of sounds

like an impossible problem. It's just shy of impossible, which is really cool. And, you know, to solve it, we actually have to go to, like, some of the deepest places of the earth. My name's Prisca Cushman, and I'm a professor of physics at the University of Minnesota. So I talked to this physicist, Prisca, who I have to say, like, in one of our conversations on video chat, like, two of her pet birds just, like, flew into the shot, and one landed on each of her shoulders, and she just continued the interview.

Really? Yeah. I won't call her a pirate, but she's at least this really cool explorer. She's working on this experiment at the bottom of a mine. You get togged up in a suit with a hard hat and a utility belt, and you get into a large, enormous elevator that takes you down with of course all the rest of the miners. The miners get off before the scientists and actually because the mine is so deep below ground it's actually warm down there because you know it's closer to the center of the earth. And then you exit into a dusty and hot environment. You're there in a regular mind drift.

So you have to walk about a kilometer to get to the laboratory itself. The really hard thing about finding dark matter is that it just passes right through normal matter. And the thing is, most of our science, or all of our scientific equipment is made out of normal matter. So it's like trying to catch like a ghost baseball with

a normal mitt. There are billions and trillions of these dark matter particles coming through the earth, coming through you and me, coming through our detector all the time. But mostly they don't interact with the detector. Mostly they just go right through it because they're so weakly interacting. But, and this is the hope, every once in a while, a dark matter particle might just, like, nudge a little bit of a nucleus of some atom of normal matter. So that the crystal within which that nucleus is gives a tiny little shiver that we can actually detect.

So the metaphor here, and this is, like, really simplified, is, like, they've created this extremely subtle bell. If you push one of the nuclei out of place, it's like giving a little tap on the bell. And that tap is so faint that it is almost impossible to hear when you're listening to all the other taps of all the other particles hitting it. So that's why they've gone deep underground. This thing is shielded from the cosmic radiation that comes from space, that comes from our sun. And there's just this beautiful patience to it of just kind of waiting and listening and hoping that the most common

source of matter in the universe will make itself apparent to us one day. And have they found the particle yet? No. I really did get into this business because I thought I would be detecting this within five years. And it's been almost 20 years for her and still nothing. I guess I'm less sanguine about the possibility that I'll discover it in five years. And it's been almost 20 years for her and still nothing. I guess I'm less sanguine about the possibility that I'll discover it in five years.

You know, there are experiments all over the world trying to detect dark matter. And they're even trying to create it at the Large Hadron Collider, the big particle collider in Europe. And no one has found anything. I gotta say, I mean, I get that all these scientists are looking for this all over the world,

but what if it's just not out there? I mean, it's invisible, it's untouchable. We've been looking for it for decades. What if there's just another way to go here? Like, when I was talking to Ashley, the science writer, for the first half of this episode, I kept thinking about the choice Vera Rubin had out there in the desert, that there were these two possible explanations for what she saw. Either Newton's law of gravity is wrong,

or we have no clue what's going on at the outer edges of the galaxy. Astronomers basically chose the second option, right? That there's tons of dark matter out there. It explains why these stars are moving too fast. But we never really tackled that first option, like reworking gravity. So I mean, like, instead of looking for this invisible particle that sounds kind of like a fantasy, what if we just kind of tweaked the laws of gravity? This is possible. Really? It makes a lot of the physicists, particularly the ones I spoke to in learning about this, kind of uncomfortable. They don't want to just throw out all these great observations that they've been making.

But at the same time, they know this other door, this idea that maybe dark matter is something of a mirage created by gravity, something we don't understand about gravity. They know that door isn't closed. I spoke to this other physicist, Priya Natarajan, who just loves thinking about these big picture problems in science. And she told me that the split to either find a physical thing or to rethink our basic assumptions. It's happened before.

In the mid-1800s, we were mapping the orbit of Uranus, which takes a long time to go around the sun, right? And what was found was that the actual shape of the orbit was slightly different from Newton's laws predictions. That was a huge problem. Like, Newton's laws? Not supposed to be broken. But Urbain Laverrier, an applied mathematician from France, realized, aha, there's an interesting possibility,

which is that maybe we are missing an observational fact. There is another planet beyond Uranus. Some source of gravity pulling on it, making this wobble. And given the wobble, given the departure of Uranus from Newton's laws, he was able to predict exactly where this planet ought to be. And, you know, lo and behold, scientists found this planet and they called it Neptune. The funny thing is, not too long after that, a similar thing happened. There was an anomaly in the orbit of Mercury.

And Urbain Laverrier said, it's the same explanation. You are missing, perhaps, a planet that lies between the sun and Mercury. And he called it Vulcan. They searched for Vulcan. I saw this little story in the New York Times from 1876. Like, please, if you can, look for Vulcan. Look for this planet crossing the sun. But they never found Vulcan.

And that anomaly remained. It actually turned out like our theory of gravity needed to be updated. Einstein needed to come along. He told us that massive objects like the sun actually bend space around it. Einstein needed to come along. He told us that massive objects like the sun actually bend space around it. And his explanation solved the problem and explained why Mercury's orbit was where it was. So we really needed a reconceptualization of gravity. Huh. So on the one hand, we could just find dark matter like we found Neptune. Yep.

But it's possible that we just need to update our theory of gravity again, which would mean there's no dark matter, just like there's no Vulcan? Yeah, there's no such thing as planet Vulcan. And, you know, like, we might not ever find the dark matter particle either. There may not be resolution, because inherent to the nature of science is the fact that whatever we know is provisional and, you know, it is apt to change. So I think this is what, you know, motivates people like me to continue doing science, is the fact that it keeps opening up more and more questions. Nothing is ultimately resolved. So what does that mean? I mean, is the idea

that we can just never know anything about dark matter for sure? No, I think the lesson is more like knowledge is a really hard one. But part of the process of that is trusting our observations. People are looking for dark matter because our observations tell us it's there to find. And there's a lot of evidence that it is there to find. Like more than stars moving fast on the edge of galaxies? Yeah. So we definitely see the stars moving fast at the edge of galaxies. That's a huge piece of evidence. But we also see evidence of dark matter in these kind of bubbles in space, like matter can distort the space around it. And we see light actually bending around dark matter.

We can also create these maps of where dark matter is in the universe by looking at where it bends light. There are these flows of dark matter, and then there are regions where dark matter filaments intersect. That's where gas falls in, cools, forms stars, and you form galaxies. Dark matter is the scaffolding of our universe. It doesn't just hold galaxies together and keep stars from flinging apart. It's why galaxies are where they are in the first place. You know, when we look into the night sky and we see galaxies, dark matter is the reason why we see what we see.

It would be correct to say that dark matter actually shapes the entire visible universe. Okay. Let me see if I can put this all together. Go for it. Vera Rubin kicked all this off by saying, you know, stars are moving way too fast and we need an explanation. Yes. And the basic idea now is that it's dark matter because that could provide all the extra gravity we need. Yeah, dark matter is that source of mass. But it's still technically possible that we could just like rewrite gravity to explain this all away, kind of pull an Einstein. I mean, you try pulling an Einstein. Yeah, technically it's

possible, but there's a lot more evidence on the side of dark matter. You can see the way it bends light. It helps us explain how galaxies formed. And, you know, at the end of the day, yes, we are still missing the key piece of evidence that dark matter is real. We haven't found the particle. Right. But that doesn't take away from everything else we know about it. It's like you're on a beach. You have a lot of sand dunes that kind of form. And so we are in a situation

where we are able to understand how these sand dunes form, but we don't actually know what a grain of sand is made of. You know, Noam, I know you were saying that the dark matter kind of felt made up. Do you still feel that way? I guess hearing this, I can get behind the idea that we don't actually have to know every tiny detail of a thing in order to understand, you know, how it works and how it affects the universe in all these ways. And I can see why all these scientists are spending so much time looking for something that has the

potential to be so important. Yeah, dark matter is a lot to accept. And you don't have to be perfectly confident in it because, you know, we don't have the perfect evidence. So like, how much do we have to accept? Like, I know there's a lot of dark matter out there, but do we know exactly how much there is? It's believed there's five times the amount of dark matter compared to normal matter in the universe. It's like funny even calling it normal matter when most of the matter in the universe, the vast majority of the matter in the universe is dark matter. And, you know, this whole time we've been talking about dark matter like it's one thing, but it doesn't have to be.

Like there could be this whole kind of ghost universe. I mean, particle physicists are really playing with this really interesting idea of an entire dark sector. Like an entire set of particles that are mirrored with the particles that we know about. It's like there's this kind of shadow universe that we don't have access to that is made up of different components that kind of exists,

like as a ghost enveloping our galaxies. We don't know what more, if we keep pulling on this thread of dark matter, what more we'll find behind that veil, in that abyss. It really just feels like that scene in Men in Black with all the marbles, you know? Or like, imagine like looking up at the stars and feeling so tiny, but you know, like times a million, like even all those stars out there are insignificant compared to dark matter. Everything we know, everything we see, everything

we've cataloged as being the universe is really only a tiny sliver of the universe. And that's just humbling. Oh, totally. I mean, I think it gives you intellectual and kind of epistemic humility, right? That we are simultaneously like super insignificant, you know, tiny, tiny speck of the universe. But on the other hand, right, we have like brains in our skulls that are like these tiny gelatinous cantaloupes

and we have figured all of this out. We have like brains in our skulls that are like these tiny gelatinous cantaloupes. And we have figured all of this out. In the grand scheme of the cosmos, you know, we're just like tiny witnesses who are here for a speck. Our lifespans are tiny, not even a wink of the eye, as it were. You know, it's something that should make us all feel really humble as well as be in total awe. The only thing I can think about is the feeling that makes me feel is, have you ever hiked the Grand Canyon, like to the bottom? Not all the way to the bottom, no.

So you get to the Grand Canyon. It looks enormous. You're at the rim. It looks like this like oil painting. Like it's so huge. But then as you start to descend into it and get towards the bottom, it only starts to look bigger. Like you realize that like little details that you saw at the top are actually huge, like craggy rock faces that descend hundreds and hundreds of feet. And you just feel so small.

And I love these moments of, like, realizing the questions are so profound and so big. You know, there's a sense of optimism in a question, right? It makes you feel like we can know the answer to them. We can fill in a little bit more of the hole of our ignorance. I feel like this is exactly Vera Rubin's story. You know, like, she got us here to the edge of this canyon. And honestly, this is the kind of thing that science doesn't always celebrate. I mean, Vera Rubin was sort of overlooked in her lifetime. She never won a

Nobel Prize. But really, she's the one who got us started on this path. Yeah. And we don't know what we're going to find down this path. This whole rich discussion we've been having about dark matter and what it is and what it could be, it's all because of her. And it's all because she pointed to this big blank spot of what we don't know.

Thanks for listening to Unexplainable. We're thinking about making more of these, about how we still don't really know how the nose works, what aliens might sound like, or why lightning sometimes shows up as a ball? What? Let us know if there's an unexplainable science mystery you might want us to look into and let us know what you thought of this episode.

You can tweet at me. I'm at Ramis Vroom. You can tweet at Today Explained. It's at today underscore explained. Email works too. Todayexplained at vox.com. Please don't email me. Today's episode of Unexplainable

was reported and produced by Brian Resnick, Bird Pinkerton, and Noam Hassenfeld, who wrote the music for it too. The episode was edited by Amy Drozdovska and Liz Kelly Nelson with some clutch pointers from Eliza Barkley and Alison Rocky. Afim Shapiro did the mixing and sound design. Thanks to Katie Mack, Catherine Zurich, Alan Lightman, and David Dvorkin. The archival tape in this episode came from David and the American Institute of Physics. If you want to learn more about Vera Rubin, Ashley Yeager is working on a biography of her,

which comes out next fall. It's called All the Matter We Cannot See. Today Explained is part of the Vox Media Podcast Network. We're taking a few days off. Back Monday.