Dark collision.

Scientists have long suspected that invisible stuff called dark matter holds galaxies together as they whiz through space. But, because no one can see dark matter, they haven't been able to prove that it exists.

Recently, astronomers spotted a rare event: two large clusters of galaxies that crashed into each other as they cruised the cosmos. By studying the collision carefully, scientists now say that they have found the first evidence of dark matter in the universe.

The scientists used a special method called gravitational lensing. The gravity of a massive object, whether visible or invisible, changes the direction in which light travels. By observing and measuring this light, researchers can figure out the location and even size of the object, whether a star, galaxy, or cloud of gas, responsible for the bending.

The observers focused on a galaxy cluster called 1E0657-56. They found that the gravity of something invisible and extremely massive had bent light coming from more-distant galaxies visible in the background.

In fact, they detected two large, separate clumps. One of the clumps, the researchers say, is made of ordinary matter, consisting of hot gases. The other clump is made of dark matter. Normally, ordinary and dark matter would be together in the same clump.

But why would dark matter separate from ordinary matter? During the impact, the hot gases of one galactic cluster slowed down the hot gases of the other. In contrast, because dark matter from one galaxy passes right through another galaxy's dark matter, the dark matter wasn't slowed by the impact.

So, the scientists could detect the two types of matter as separate clouds.

"This proves in a simple and direct way that dark matter exists," says Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics.

http://www.sciencenewsforkids.org/articles/20060830/Note2.asp From Science News for Kids Aug. 30, 2006.