First Dark Matter Evidence Found

Aug. 21, 2006 —Astronomers announced today they now have the first direct evidence of dark matter — although they still have only an inkling of what the elusive stuff is made of.

The unprecedented observations come from careful weighing of gas and stars being flung about in the most violent and massive collision in the known universe.

It’s a tiff between two clusters of galaxies in what’s collectively called the Bullet Cluster, which has caused stars and dark matter from different galaxies to tear past each other while the more widely distributed interstellar gases collide and slow.

"All the matter in a typical galaxy occupies the same space," said astrophysicist Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics, speaking at a NASA press conference. "In this case the gas and galaxies are separated in space. Galaxies flew through each other but their gas clouds didn’t so easily."

Visualize, for instance, a cosmic million-mile-per-hour collision between two vast wads of raisin oatmeal – with stars and dark matter comprising the raisins and oats representing the gases. The raisins would shoot through with few direct raisin-on-raisin hits, while the oats would get stuck in a patch in the middle.

The result is different patches of space: one with lots of hot colliding gas and two others on either side with all the dark matter and stars in visible galaxies.

The astrophysicists know the visible stars still have the dark matter with them because they weighed the mass in the starry patches by measuring how those patches bend the light from far more distant objects. The more a starry region bends light, the more massive it is.

In this case, the starry areas in the colliding clusters have far more mass than can be accounted for by visible stars or by interstellar gases – since the stars left the gases behind. The only thing left to explain it is dark matter.

"This proves in a direct and simple way that dark matter exists," said Markevitch.

The discovery was made using a suite of observations from the orbiting Hubble Space Telescope and Chandra X-Ray Observatory, along with the ground-based European Southern Observatory's Very Large Telescope and the Magellan telescope. A paper on the discovery will be published in the next issue of Astrophysical Journal Letters.

Until now the existence of dark matter was inferred by the fact that galaxies have only one-fifth of the visible matter needed to create the gravity that keeps them intact. So the rest must be invisible to telescopes. In a word, that unseen matter is "dark."

The observations of the Bullet Cluster, officially known as galaxy cluster 1E0657-56, do not explain what dark matter is. They do, however, provide one solid little hint, said Douglas Clowe, a researcher at the University of Arizona in Tucson.

"We can place some constraints on dark matter particles," said Clowe. It appears that dark matter particles, whatever they are, behave more like the raisins than the oatmeal – they are either widely spaced, like stars, or have some other way of avoiding collisions with each other.

It’s a small clue, said Clowe, but seeing it play out in the Bullet Cluster makes it an unusually solid clue for what’s so far proven to be the most mysterious stuff in the universe.

"The great news about this is that it shows once and for all that dark matter exists," said physicist Sean Carroll of the University of Chicago. And that means, he said, there’s less need to tweak Einstein’s laws of gravitation to explain what’s seen in galaxies.