Enlightened: dark matter spotted after cosmic crash.

An intergalactic collision is providing astronomers with a giant payoff: the first direct evidence of the invisible material that theorists say holds galaxies together and accounts for most of the universe's mass.

For some 70 years, cosmologists have agreed that theories of gravity account for observations in Earth's solar system but fail on a larger scale. For example, if those theories held throughout the universe, objects on the outskirts of the Milky Way would rotate more slowly than those toward the center. But they don't.

Scientists have offered two competing explanations of this discrepancy. The first is that an invisible substance called dark matter accounts for 90 percent of the universe's mass and gravity. Although scientists don't know what dark matter consists of, they propose that it keeps each galaxy intact (SN: 8/13/05, p. 104).

The second explanation says that dark matter doesn't exist and that traditional models of gravity simply need modification.

To search for dark matter, Douglas Clowe of the University of Arizona in Tucson and his colleagues used several telescopes and observatories to image an unusually energetic collision between two galaxies that occurred 100 million years ago.

Normally, as galaxies travel through the universe, gravity keeps dark and ordinary matter close together, so the invisible substance can't be distinguished. During a galactic merger, however, hot gases from one galaxy bump into hot gases in the other and both galaxies are slowed by a force similar to wind resistance. But dark matter from one galaxy, in theory, passes right through another galaxy's dark matter (SN: 4/23/05, p. 264).

"Dark matter particles don't experience the same type of drag that slows down gas clouds," says Clowe.

His team used a technique called gravitational lensing to locate the main mass in the aftermath of the collision (SN: 5/20/00, p. 332). If dark matter didn't exist, all the mass would have been lumped together with the gases. Instead, the researchers found most of the mass in clumps that appeared to have whizzed past the hot gases.

Only a theory of gravity that includes dark matter can explain the separation, Clowe's team argues in an upcoming Astrophysical Journal Letters.

"This proves in a simple and direct way that dark matter exists," says coauthor Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "It puts to rest the remaining doubt that cosmologists have had until now"

The matter separation caused by the collision is "mind-boggling," says cosmologist Michael Turner of the University of Chicago. However, he adds that the researchers can't rule out alternative theories, in part because the models from them are so inconsistent.

Alternative models will have a hard time challenging the new finding, maintains astrophysicist Katherine Freese of the University of Michigan in Ann Arbor. "It's going to make it tough for anybody to compete," she says.

Down the line, the observation might give researchers important insights into intergalactic mergers, says Turner. "It's kind of like a cosmic centrifuge," he says.