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.
