What Is Dark Matter? A New Clue!

  • By Greg Kestin
  • Posted 03.09.18
  • NOVA

In radio waves from the dawn of the universe, scientists may have found the first evidence that dark matter—which scientists know little about—is made of particles. Now, decades since dark matter was discovered through the effects of its gravity, this discovery may help researchers figure out how to search for it. Find out how in this episode of What the Physics?!

Running Time: 03:38


What Is Dark Matter? A New Clue!

Published March 9, 2018

Greg Kestin: A team of physicists just announced that they may have discovered the first evidence that dark matter is actually made of particles. They found the evidence in radio waves that came from the dawn of the universe. And their radio wave detector doesn't look that much more sophisticated than a ham radio.

Most of the matter around you--and matter that's in you--is invisible. It's dark... It's dark matter. The only reason we know it exists is because, like anything with mass, its gravity pulls on things that we can see. Its gravity is so strong that it can make a normal patch of sky look like this. Something really massive is bending the light around this galaxy, and for it to look this weird--to get that much lensing--you'd need mass that's 20 times the mass of the galaxy itself.

So, we know what dark matter does, but we don't know what dark matter is. And there are a bunch of wild ideas. Dark matter could be a bunch of black holes; it could be supersymmetric particles that mirror our normal, everyday particles; it could not exist at all and maybe gravity's just much stranger than we expected. There's really been no way to tell which one of these is right...until, maybe, now.

Dark matter seems to almost live in a parallel universe, meaning, it's right here, right now; it's going through me, it's going through you; you can't see it, you can't feel it, but it's there. So, how do you look into that dark realm?

Theoretically, there are a few ways we might be able to catch a glimpse of what it is. Particle physicists at the Large Hadron Collider are trying to make it by smashing protons together. Astrophysicists are trying to see dark matter annihilate into light. But radio astronomers may have stumbled onto dark matter through an unexpected route.

A group from Arizona State was looking for a signal in the sky. They were trying to find the moment the first stars "turned on," and the universe went from dark to light. Basically, when the first stars "turned on," it set off a reaction that made a bunch of hydrogen act like a cloud. A cloud that absorbed a very specific wavelength of light. That is, the hydrogen started to block some of the light that had a wavelength about that big.

After about 10 years, they finally found that dip. But there was something else.

Imagine you're just chillin' on the beach, looking up at the sky, when, just like the forecast said, some clouds roll in. But then, all of a sudden, it's much darker and much colder than you expected.

That's kind of like what happened to the scientists looking for the dawn of the universe. They found the clouds that were created by the first stars, but it was much darker than they expected. And that suggested that this gas, the cloud, was likely much colder than they expected, too.

So something was probably cooling down that gas. Maybe something invisible...or dark. The discoverers' best ideas is that it's dark matter particles that are moving really slowly. That's cold dark matter. And this cold dark matter would have been bumping into that gas, cooling it down. Other ideas that people have for what dark matter is, like black holes, couldn't have cooled down the gas like this. So, maybe we finally have some evidence that dark matter is particles.

There are a number of other experiments that are now tuning their instruments to that frequency. So we may know a lot more about dark matter very soon.



Host, Producer
Greg Kestin
Greg Kestin
Samia Bouzid
Greg Kestin
Samia Bouzid
Scientific Consultants
Douglas Finkbeiner
Editorial Input from
Ari Daniel
Julia Cort
Greg Kestin
Animation and Editing
Greg Kestin
Special thanks
Entire NOVA team
From the producers of PBS NOVA © WGBH Educational Foundation
Funding provided by FQXi

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