Deciphering the Matter of Dark Matter

Astronomers say they have proven that a mysterious material that fills our universe, called dark matter, actually exists. Dark matter is dark, so it can't be seen directly. But it is thought to account for 90 percent of the matter in our universe. Now, astronomers studying the collision of two galaxies say that cosmic crash reveals the presence of dark matter. But what the matter is actually composed of remains one of the biggest mysteries in our universe.

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Today astronomers say they finally have proof that the universe is full of a mysterious substance nobody has ever seen. It's called dark matter. Theorists have believed for many years that there is much more dark matter in the universe than there is ordinary stuff like atoms.

No one has seen dark matter and that's still the case, but as NPR's Richard Harris reports, some astronomers are now dead certain that it is out there.

RICHARD HARRIS reporting:

Astronomers confront a big mystery when they stare out into space. Thousands of galaxies clump together to form super clusters of galaxies. Gravity must have drawn all that stuff together, but when you add up the mass of all the stars and all the gas and all the other visible material in those galaxies, there simply isn't enough stuff there to explain the gravitational attraction. Not by a long shot.

So Doug Clowe at the University of Arizona says most astronomers long ago decided that most of the matter in the universe must be invisible and exotic dark matter.

Mr. DOUG CLOWE (University of Arizona): As a result, astronomers have long been in the slightly embarrassing position of having to explain their observations using something that we didn't know actually existed.

HARRIS: Now Clowe and his colleagues say they have at long last been able to relieve themselves of that embarrassment. They say they can now prove that dark matter exists.

Maxim Markovich at the Harvard Smithsonian Center for Astrophysics says the story starts a few years ago, when the Chandra X-ray Observatory discovered two enormous clusters of galaxies colliding with one another out in the far reaches of the universe.

Mr. MAXIM MARKOVICH (Harvard Smithsonian Center for Astrophysics): And this is the most energetic event known in the universe since the Big Bang.

HARRIS: Each cluster contains thousands of galaxies along with vast clouds of gas. When the clusters slammed together at more than a million miles an hour, the galaxies simply shot past one another like bullets at dizzying speeds. But the gas clouds collided and came to a screeching halt at the site of the collision. Nature had performed a rare experiment to separate galaxies from their surrounding gas clouds.

Now, 90 percent of the normal matter in this super cluster turns out to be the gas clouds themselves. The galaxies are puny in comparison. But when Markovich and Clowe measured the tug of gravity from the gas cloud and then from the galaxies, they were in for a shock. The seemingly tiny galaxies actually had a much greater tug of gravity than the gas cloud did.

Mr. CLOWE: Well, the only way that this can occur is if there is an additional source of matter which is sitting up with the galaxies that is much, much more massive than that hot gas.

HARRIS: In a teleconference, Maxim Markovich asserted there is only one explanation.

Mr. MARKOVICH: And this proves in a simple and direct way that dark matter exists.

HARRIS: Now this will not come as a shock to any astronomer. Most have been operating under the assumption that dark matter must exist. But Martin White at the University of California Berkeley says it's certainly important scientifically to have this kind of iron clad evidence.

Mr. MARTIN WHITE (University of California): In some ways they got lucky that there existed such an object in the universe that they could use to make this measurement, but then it was very clever to exploit it the way they did.

HARRIS: White notes that some theorists have been pushing an alternate explanation for dark matter. They've argued that maybe gravity itself starts doing goofy things at huge scales.

Maybe. But White says this finding shows dark matter must be out there, and alas, it doesn't really get us any closer to answering the biggest mystery about dark matter, which is what is it exactly?

Mr. WHITE: We don't really know, but we think it's probably some kind of sub-atomic particle that we haven't seen yet, which we presume that there must be quite a few.

HARRIS: And people are looking hard for those in particle accelerators here on earth.

Richard Harris, NPR News.

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