Scientists Illuminate the Nature of Dark Matter

More than 20 years ago, scientists discovered that most of the matter in the universe isn't made up of ordinary atoms but rather invisible material dubbed dark matter. New observations of nearby galaxies suggest that dark matter only comes in enormous clumps of particles that race through space faster than fighter jets.

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Astronomers say they have some intriguing new clues about one of the biggest mysteries of the universe. More than twenty years ago, scientists discovered that most of the matter in the universe is not made up of ordinary atoms, it's something invisible called dark matter.

New observations of nearby galaxies suggest that dark matter only comes in enormous clumps of particles that race through space faster than fighter jets. NPR's Richard Harris has the story.

RICHARD HARRIS reporting:

Of all the big mysteries in the universe, dark matter is surely one of the most alluring.

Professor GERRY GILMORE (Experimental Philosophy, University of Cambridge): Well, it's impossible not to be interested in dark matter.

HARRIS: Gerry Gilmore is a professor of experimental philosophy at the University of Cambridge. He's been fascinated by dark matter for his entire career. He says our very existence depends on it.

Professor GILMORE: The Milky Way is full of dark matter, and it is the reason the sun is still in the Milky Way. It's the reason there are stars in the sky. All the stars in our sky would fly off into outer space if the weight of the dark matter wasn't holding them in.

HARRIS: By its very nature, nobody can see dark matter. But that hasn't stopped scientists from trying to infer what it's made of. At first they thought it was made of ordinary atoms, maybe dead planets or black holes, simply stuff that couldn't be seen because it's not glowing like stars. That proved not to be the case, Gilmore says. Instead, dark matter seems to be made up of some sort of exotic particle.

Professor GILMORE: The simplest explanation of these dark matter particles is that they were all created in the Big Bang, and then therefore they've just been expanding and cooling down, and moving slower and slower as the universe got bigger and bigger.

And so the simplest explanation was they're really very slow moving, moving at millimeters per second.

HARRIS: But Gilmore's new measurement suggests that is not in fact the case.

He's part of a team of astronomers that use telescopes around the world to study the motion of stars in nearby galaxies. The long and short of that study is that dark matter particles are moving at the speed of supersonic fighter jets.

Professor GILMORE: And so they're moving about a million times faster than expected.

HARRIS: Now, fast moving particles can't be packed close together. That's why rapidly moving water molecules make up a diffused gas: steam. Likewise, these super fast particles can't be jammed into a small space. Gilmore says the smallest possible clump of dark matter seems to be so vast it would take you a thousand years to cross it, traveling at the speed of light.

Professor GILMORE: You just don't see the stuff compressed up on smaller scales. Now that's still an astronomical number, but it's very, very small compared to what it might have been, or very, very big compared to what most theories had predicted.

HARRIS: Obviously these clues don't reveal the true nature of dark matter, but they do appear to eliminate some theories and favor other ideas. Gilmore hasn't actually published these results anywhere, so other astronomers can't really evaluate how strong his new evidence is.

Astronomer Mario Mateo, at the University of Michigan, has been making similar measurements. He says there are so many uncertainties in these kinds of studies it's hard to be confident in the results. Even so, he is intrigued.

Professor MARIO MATEO (Astronomy, University of Michigan): We are actually starting to say something about the underlying nature of the particles, which I, you know, I find is fascinating. When you're not doing it in a lab, here on the earth, you're doing it thousands of light years away.

HARRIS: It is just possible that more clues about dark matter will eventually come from a lab on earth. Next year, a giant particle accelerator in Switzerland is supposed to be up and running. Experiments there could at long last provide some tangible clues about this unearthly material; which, after all, seems to make up 80 percent of our universe.

Richard Harris, NPR News.

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