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Scientists are still working on theories that might help explain what the vast majority of our universe is made of.
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Paul J. Steinhardt, Neil Turok, The Endless Universe.

The phrase “dark matter” used to just be a catch-all term for anything astronomers couldn’t see, but they knew had to be there, like a nebula that didn’t emit light.

But these days, it’s something more specific: it’s the 84% of matter in the universe that we don’t understand. And all our best ideas to explain it have run straight into walls.

Astronomers discovered dark matter back in the 1970s, based on the way galaxies moved and rotated. They went way too fast for the gravity from the stars and gas astronomers saw to be the only thing keeping those galaxies from flying apart. It seemed like there must be extra, invisible matter that we can’t see or detect directly because it didn’t produce or absorb or reflect light.

All we see is its gravitational pull. For a while, some people doubted the observations, and others started thinking maybe we were wrong about how gravity works. But at this point, most cosmologists agree that we just have to accept that there’s invisible matter out there making up the majority of the universe.

Because the evidence keeps piling up. Everything, from the evolution of the entire universe to the leftover signals from the Big Bang, seems to scream, “DARK MATTER.” But despite all the evidence that exists, we still don’t know what dark matter is made of. There are some scientists still trying to subtly tweak the math we use to describe gravity so it fits all these observations, without assuming that there’s a bunch of invisible matter out there.

They work on what’s called  MOND: MOdified Newtonian Dynamics. They’ve been able to use their reworked laws of physics to explain some of the things that seem like evidence for dark matter. But objects like the Bullet Cluster, a pair of colliding clusters of galaxies where there seems to be dark matter completely separated from regular matter, really took the wind out of MOND’s sails, at least for now.

Most cosmologists accept that dark matter is out there, and they want to figure out what it is. By looking at different models for how the universe developed after the Big Bang, they’ve found lots of different kinds of objects that could be plentiful and dark enough to account for the observations. There are two main lines of thought on this, which we’ll just call “old physics” and “new physics”.

The “old physics” crew wants to explain dark matter with stuff we already know exists. Some of them think that dark matter is literally that: Regular matter that’s dark, like black holes or neutron stars or failed stars known as brown dwarfs. All of these are called MACHOs, with an M. Not the delicious snack. It stands for MAssive Compact Halo Objects. MACHOs produce little, if any, light of their own, so it would make sense if all we could see was their gravity. Thing is, when researchers have looked more carefully with sensitive telescopes, they haven’t seen anywhere near enough MACHOs to explain all of the dark matter out there.

Other people in the “old physics” camp used to think dark matter was made of neutrinos: ghostly subatomic particles that come out of processes like the nuclear reactions that happen in stars. Neutrinos don’t produce light, and they barely interact with other matter at all, which made them a great candidate for dark matter. They also have almost no mass, but even though each individual neutrino is incredibly light, the mass would add up, as there are a lot of them. If there are enough neutrinos out there, they could explain all that extra gravity. But there’s a problem: the neutrinos we know about move way too fast to clump together the way we’ve seen dark matter clumped together in huge structures like galaxies.

Which brings us to the “new physics” camp: scientists who are looking for new kinds of particles that predict what we see. A lot of them are looking for WIMPs, or Weakly Interacting Massive Particles, so named because they only interact very weakly with normal matter. Which would explain why it is so hard for us to detect them. There are lots of types of WIMPs that researchers think could be dark matter, and they keep doing experiments to look for them. But they’ve never found any, and with every experiment, they keep ruling out more and more types of WIMPs.

Another possibility is that we’re missing something about neutrinos. Some scientists think there might be a new kind of super-heavy neutrino that we just haven’t detected yet, because you wouldn’t need as many heavy neutrinos to account for dark matter as you would light neutrinos. Sort of like how it only takes just a few large chips with lots of toppings to fill you up instead of just a bunch of little tiny chips. I don’t know how we got back to nachos. I’m hungry? Anyway, if there aren’t as many heavy neutrinos out there to begin with, then it would make sense that we haven’t spotted any yet.

There are also “new physics” people who think axions might be the key. Axions are particles that were predicted back in the 1970s to solve a completely unrelated problem in physics. But like neutrinos, they’d also be almost weightless and hard to detect, and some models suggest that some or even all dark matter could be made of axions.

Then there are the really creative physicists who are working on an idea called supersymmetry, where every particle in the universe has a kind of heavy twin, and one of those twins would be dark matter. There are even some researchers who think we’re seeing the effects of parallel universes messing with ours. But, again, there is no direct evidence for any of these new ideas. No one has seen a super-heavy neutrino, no one has seen an axion, no one’s seen a supersymmetric twin, and no one has seen a parallel universe. Experiments have ruled out the simplest models of heavy neutrinos and supersymmetry, and they’ve ruled out axions as an explanation for at least the majority of dark matter out there.

So for now, we’re stuck. Scientists around the world are working as hard as they can to understand dark matter better, from the ones staying up all night at the telescope to the ones staying up all night at the blackboard. But as it stands right now, the vast majority of the matter in the universe remains a complete mystery.

This episode was inspired by and brought to you by We Have No Idea, a new book by Jorge Cham and Daniel Whiteson. Mixing cartoons and humor with serious science, We Have No Idea explores the mysteries of dark matter and lots of other open questions about the universe. Like “How many dimensions are there?” or “Why aren’t we made of anti-matter?” The book is available now, and you can get your copy at