Dark matter holds our universe together. No one knows what it is.

Dark matter, unexplained.

If you go outside on a dark night, in the darkest places on Earth, you can see as many as 9,000 stars. They appear as tiny points of light, but they are massive infernos. And while these stars seem astonishingly numerous to our eyes, they represent just the tiniest fraction of all the stars in our galaxy, let alone the universe.

The beautiful challenge of stargazing is keeping this all in mind: Every small thing we see in the night sky is immense, but what’s even more immense is the unseen, the unknown.

I’ve been thinking about this feeling — the awesome, terrifying feeling of smallness, of the extreme contrast of the big and small — while reporting on one of the greatest mysteries in science for Unexplainable, a brand new Vox podcast about unanswered questions in science. You can listen to the episode below (and subscribe wherever you listen to podcasts!).

It turns out all the stars in all the galaxies, in all the universe, barely even begin to account for all the stuff of the universe. Most of the matter in the universe is actually unseeable, untouchable, and, to this day, undiscovered.

Scientists call this unexplained stuff “dark matter,” and they believe there’s five times more of it in the universe than normal matter — the stuff that makes up you and me, stars, planets, black holes, and everything we can see in the night sky or touch here on Earth. It’s strange even calling all that “normal” matter, because in the grand scheme of the cosmos, normal matter is the rare stuff. But to this day, no one knows what dark matter actually is.

“I think it gives you intellectual and kind of epistemic humility — that we are simultaneously, super insignificant, a tiny, tiny speck of the universe,” Priya Natarajan, a Yale physicist and dark matter expert, said on a recent phone call. “But on the other hand, we have brains in our skulls that are like these tiny, gelatinous cantaloupes, and we have figured all of this out.”

The story of dark matter is a reminder that whatever we know, whatever truth about the universe we have acquired as individuals or as a society, is insignificant compared to what we have not yet explained.

It’s also a reminder that, often, in order to discover something true, the first thing we need to do is account for what we don’t know.

This accounting of the unknown is not often a thing that’s celebrated in science. It doesn’t win Nobel Prizes. But, at least, we can know the size of our ignorance. And that’s a start.

But how does it end? Though physicists have been trying for decades to figure out what dark matter is, the detectors they built to find it have gone silent year after year. It makes some wonder: Have they been chasing a ghost? Dark matter might not be real. Instead, there could be something more deeply flawed in physicists’ understanding of gravity that would explain it away. Still, the search, fueled by faith in scientific observations, continues, despite the possibility that dark matter may never be found.

To learn about dark matter is to grapple with, and embrace, the unknown.

The woman who told us how much we don’t know

Scientists are, to this day, searching for dark matter because they believe it is there to find. And they believe so largely because of Vera Rubin, an astronomer who died in 2016 at age 88.

Growing up in Washington, DC, in the 1930s, like so many young people getting started in science, Rubin fell in love with the night sky.

Rubin shared a bedroom and bed with her sister Ruth. Ruth was older and got to pick her favorite side of the bed, the one that faced the bedroom windows and the night sky.

“But the windows captivated Vera’s attention,” Ashley Yeager, a journalist writing a forthcoming biography on Rubin, says. “Ruth remembers Vera constantly crawling over her at night, to be able to open the windows and look out at the night sky and start to track the stars.” Ruth just wanted to sleep, and “there Vera was tinkering and trying to take pictures of the stars and trying to track their motions.”

It wasn’t that everything we knew about matter was wrong. It was that everything we knew about normal matter was insignificant.

Not everyone gets to turn their childlike wonder and captivation of the unknown into a career, but Rubin did.

Flash-forward to the late 1960s, and she’s at the Kitt Peak National Observatory near Tucson, Arizona, doing exactly what she did in that childhood bedroom: tracking the motion of stars.

This time, though, she has a cutting-edge telescope and is looking at stars in motion at the edge of the Andromeda Galaxy. Just 40 years prior, Edwin Hubble had determined, for the first time, that Andromeda was a galaxy outside of our own, and that galaxies outside our own even existed. With one observation, Hubble doubled the size of the known universe.

By 1960, scientists were still asking basic questions in the wake of this discovery. Like: How do galaxies move?

Rubin and her colleague Kent Ford were at the observatory doing this basic science, charting how stars are moving at the edge of Andromeda. “I guess I wanted to confirm Newton’s laws,” Rubin said in an archival interview with science historian David DeVorkin.

Alan Dyer/Universal Images Group/Getty Images

The Andromeda Galaxy.

Per Newton’s equations, the stars in the galaxy ought to move like the planets in our solar system do. Mercury, the closest planet to the sun, orbits very quickly, propelled by the sun’s gravity to a speed of around 106,000 mph. Neptune, far from the sun, and less influenced by its gravity, moves much slower, at around 12,000 mph.

The same thing ought to happen in galaxies too: Stars near the dense, gravity-rich centers of galaxies ought to move faster than the stars along the edges.

But that wasn’t what Rubin and Ford observed. Instead, they saw that the stars along the edge of Andromeda were going the same speed as the stars in the interior. “I think it was kind of like a ‘what the fuck’ moment,” Yeager says. “It was just so different than what everyone had expected.”

Ingo Berg/Wikipedia

On the left, what Rubin expected to see: stars orbiting the outskirts of a galaxy moving slower than those near the center. On the right, what was observed: the stars on the outside moving at the same speed as the center.

The data pointed to an enormous problem: The stars couldn’t just be moving that fast on their own.

At those speeds, the galaxy should be ripping itself apart like an accelerating merry-go-round with the brake turned off. To explain why this wasn’t happening, these stars needed some kind of extra gravity out there acting like an engine. There had to be a source of mass for all that extra gravity. (For a refresher: Physicists consider gravity to be a consequence of mass. The more mass in an area, the stronger the gravitational pull.)

The data suggested that there was a staggering amount of mass in the galaxy that astronomers simply couldn’t see. “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,” Yeager says. It wasn’t black holes; it wasn’t dead stars. It was something else generating the gravity needed to both hold the galaxy together and propel those outer stars to such fast speeds.

“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,” Rubin later recounted. Other scientists might have immediately announced a dramatic conclusion based on this limited data. But not Rubin. She and her collaborators dug in and decided to do a systematic review of the star speeds in galaxies.

Rubin and Ford weren’t the first group to make an observation of stars moving fast at the edge of a galaxy. But what Rubin and her collaborators are famous for is verifying the finding across the universe. “She [studied] 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,” Yeager explains.

This is why people say Rubin ought to have won a Nobel Prize (the prizes are only awarded to living recipients, so she will never win one). She didn’t “discover” dark matter. But the data she collected over her career made it so the astronomy community had to reckon with the idea that most of the mass in the universe is unknown.

By 1985, Rubin was confident enough in her observations to declare something of an anti-eureka: announcing not a discovery, but a huge absence in our collective knowledge. “Nature has played a trick on astronomers,” she’s paraphrased as saying at an International Astronomical Union conference in 1985, “who thought we were studying the universe. We now know that we were studying only a small fraction of it.”

To this day, no one has “discovered” dark matter. But Rubin did something incredibly important: She told the scientific world about what they were missing.

In the decades since this anti-eureka, other scientists have been trying to fill in the void Rubin pointed to. Their work isn’t complete. But what they’ve been learning about dark matter is that it’s incredibly important to the very structure of our universe, and that it’s deeply, deeply weird.

Dark matter isn’t just enormous. It’s also strange.

Since Rubin’s WTF moment in the Arizona desert, more and more evidence has accumulated that dark matter is real, and weird, and accounts for most of the mass in the universe.

“Even though we can’t see it, we can still infer that dark matter is there,” Kathryn Zurek, a Caltech astrophysicist, explains. “Even if we couldn’t see the moon with our eyes, we would still know that it was there because it pulls the oceans in different directions — and it’s really very similar with dark matter.”

Scientists can’t see dark matter directly. But they can see its influence on the space and light around it. The biggest piece of indirect evidence: Dark matter, like all matter that accumulates in large quantities, has the ability to warp the very fabric of space.

“You can visualize dark matter as these lumps of matter that create little potholes in space-time,” Natarajan says. “All the matter in the universe is pockmarked with dark matter.”

When light falls into one of these potholes, it bends like light does in a lens. In this way, we can’t “see” dark matter, but we can “see” the distortions it produces in astronomers’ views of the cosmos. From this, we know dark matter forms a spherical cocoon around galaxies, lending them more mass, which allows their stars to move faster than what Newton’s laws would otherwise suggest.

NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland), R. Massey (Durham University, UK), Harald Ebeling (University of Hawaii at Manoa) and Jean-Paul Kneib (LAM)

This is a NASA/ESA Hubble Space Telescope image of the galaxy cluster MACS J0717.5+3745. Shown in blue on the image is a map of the dark matter found within the cluster.

These are indirect observations, but they have given scientists some clues about the intrinsic nature of dark matter. It’s not called dark matter because of its color. It has no color. It’s called “dark” because it neither reflects nor emits light, nor any sort of electromagnetic radiation. So we can’t see it directly even with the most powerful telescopes.

Not only can we not see it, we couldn’t touch it if we tried: If some sentient alien tossed a piece of dark matter at you, it would pass right through you. If it were going fast enough, it would pass right through the entire Earth. Dark matter is like a ghost.

Here’s one reason physicists are confident in that weird fact. Astronomers have made observations of galaxy clusters that have slammed into one another like a head-on collision between two cars on the highway.

Astronomers deduced that in the collision, much of the normal matter in the galaxy clusters slowed down and mixed together (like two cars in a head-on collision would stop one another and crumple together). But the dark matter in the cluster didn’t slow down in the collision. It kept going, as if the collision didn’t even happen.

The event is recreated in this animation. The red represents normal matter in the galaxy clusters, and the blue represents dark matter. During the collision, the blue dark matter acts like a ghost, just passing through the normal colliding matter as if it weren’t there.

John Wise of the Kavli Institute for Particle Astrophysics and Cosmology

(A note: These two weird aspects of dark matter — its invisibility and its untouchability — are connected: Dark matter simply does not interact with the electromagnetic force of nature. The electromagnetic force lights up our universe with light and radiation, but it also makes the world feel solid.)

A final big piece of evidence for dark matter is that it helps physicists make sense of how galaxies formed in the early universe. “We know that dark matter had to be present to be part of that process,” astrophysicist Katie Mack explains. It’s believed dark matter coalesced together in the early universe before normal matter did, creating gravitational wells for normal matter to fall into. Those gravitational wells formed by dark matter became the seeds of galaxies.

So dark matter not only holds galaxies together, as Rubin’s work implied — it’s why galaxies are there in the first place.

So: What is it?

To this day, no one really knows what dark matter is.

Scientists’ best guess is that it’s a particle. Particles are the smallest building blocks of reality — they’re so small, they make up atoms. It’s thought that dark matter is just another one of these building blocks, but one we haven’t seen up close for ourselves. (There are a lot of different proposed particles that may be good dark matter candidates. Scientists still aren’t sure exactly which one it will be.)

You might be wondering: Why can’t we find the most common source of matter in all the universe? Well, our scientific equipment is made out of normal matter. So if dark matter passes right through normal matter, trying to find dark matter is like trying to catch a ghost baseball with a normal glove.

Plus, while dark matter is bountiful in the universe, it’s really diffuse. There are just not massive boulders of it passing nearby Earth. It’s more like we’re swimming in a fine mist of it. “If you add up all the dark matter inside humans, all humans on the planet at any given moment, it’s one nanogram,” Natarajan says — teeny-tiny.

Dark matter may never be “discovered,” and that’s okay

Some physicists favor a different interpretation for what Rubin observed, and for what other scientists have observed since: that it’s not that there’s some invisible mass of dark matter dominating the universe, but that scientists’ fundamental understanding of gravity is flawed and needs to be reworked.

While “that’s a definite possibility,” Natarajan says, currently, there’s a lot more evidence on the side of dark matter being real and not just a mirage based on a misunderstanding of gravity. “We would need a new theory [of gravity] that can explain everything that we see already,” she explains. “There is no such theory that is currently available.”

NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

On the left, a Hubble Space Telescope image of a galaxy cluster. On the right, a blue shading has been added to indicate where the dark matter ought to be.

It’s not hard to believe in something invisible, Mack says, if all the right evidence is there. We do it all the time.

“It’s similar to if you’re walking down the street,” she says. “And as you’re walking, you see that some trees are kind of bending over, and you hear some leaves rustling and maybe you see a plastic bag sort of floating past you and you feel a little cold on one side. You can pretty much figure out there’s wind. Right? And that wind explains all of these different phenomena. ... There are many, many different pieces of evidence for dark matter. And for each of them, you might be able to find some other explanation that works just as well. But when taken together, it’s really good evidence.”

Meanwhile, experiments around the world are trying to directly detect dark matter. Physicists at the Large Hadron Collider are hoping their particle collisions may one day produce some detectable dark matter. Astronomers are looking out in space for more clues, hoping one day dark matter will reveal itself through an explosion of gamma rays. Elsewhere, scientists have burrowed deep underground, shielding labs from noise and radiation, hoping that dark matter will one day pass through a detector they’ve carefully designed and make itself known.

But it hasn’t happened yet. It may never happen: Scientists hope that dark matter isn’t a complete ghost to normal matter. They hope that every once in a while, when it collides with normal matter, it does something really, really subtle, like shove one single atom to the side, and set off a delicately constructed alarm.

But that day may never come. It could be dark matter just never prods normal matter, that it remains a ghost.

“I really did get into this business because I thought I would be detecting this within five years,” Prisca Cushman, a University of Minnesota physicist who works on a dark matter detector, says. She’s been trying to find dark matter for 20 years. She still believes it exists, that it’s out there to be discovered. But maybe it’s just not the particular candidate particle her detector was initially set up to find.

That failure isn’t a reason to give up, she says. “By not seeing [dark matter] yet with a particular detector, we’re saying, ‘Oh, so it’s not this particular model that we thought it might be.’ And that is an extremely interesting statement. Because all of a sudden an army of theorists go out and say, ‘Hey, what else could it be?’”

But even if the dark matter particle is never found, that won’t discount all science has learned about it. “It’s like you’re on a beach,” Natarajan explains. “You have a lot of sand dunes. 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.”

Embracing the unknown

Natarajan and the other physicists I spoke to for this story are comfortable with the unknown nature of dark matter. They’re not satisfied, they want to know more, but they accept it’s real. They accept it because that’s the state of the evidence. And if new evidence comes along to disprove it, they’ll have to accept that too.

“Inherent to the nature of science is the fact that whatever we know is provisional,” Natarajan says. “It is apt to change. So I think what motivates people like me to continue doing science is the fact that it keeps opening up more and more questions. Nothing is ultimately resolved.”

That’s true when it comes to the biggest questions, like “what is the universe made of?”

It’s true in so many other areas of science, too: Despite the endless headlines that proclaim new research findings that get published daily, there are many more unanswered questions than answered. Scientists don’t really understand how bicycles stay upright, or know the root cause of Alzheimer’s disease or how to treat it. Similarly, at the beginning of the Covid-19 pandemic, we craved answers: Why do some people get much sicker than others, what does immunity to the virus look like? The truth was we couldn’t yet know (and still don’t, for sure). But that didn’t mean the scientific process was broken.

The truth is, when it comes to a lot of fields of scientific progress, we’re in the middle of the story, not the end. The lesson is that truth and knowledge are hard-won.

In the case of dark matter, it wasn’t that everything we knew about matter was wrong. It was that everything we knew about normal matter was insignificant compared to our ignorance about dark matter. The story of dark matter fits with a narrative of scientific progress that makes us humans seem smaller and smaller at each turn. First, we learned that Earth wasn’t the center of the universe. Now dark matter teaches us that the very stuff we’re made of — matter — is just a fraction of all reality.

If dark matter is one day discovered, it will only open up more questions. Dark matter could be more than one particle, more than one thing. There could be a richness and diversity in dark matter that’s a little like the richness and diversity we see in normal matter. It’s possible, and this is speculation, that there’s a kind of shadow universe that we don’t have access to — scientists label it the “dark sector” — that is made up of different components and exists, as a ghost, enveloping our galaxies.

It’s a little scary to learn how little we know, to learn we don’t even know what most of the universe is made out of. But there’s a sense of optimism in a question, right? It makes you feel like we can know the answer to them.

There’s so much about our world that’s arrogant: from politicians who only believe in what’s convenient for them to Silicon Valley companies that claim they’re helping the world while fracturing it, and so many more examples. If only everyone could see a bit of what Vera Rubin saw — a fundamental truth not just about the universe, but about humanity.

“In a spiral galaxy, the ratio of dark-to-light matter is about a factor of 10,” Rubin said in a 2000 interview. “That’s probably a good number for the ratio of our ignorance to knowledge. We’re out of kindergarten, but only in about third grade.”