Bullet Cluster 
About 150 million years ago as seen from Earth, two clusters of galaxies rammed together. The collision stripped away most of the gas from the galaxies, forming a giant, superhot cloud. But the galaxies themselves flew right through each other. And apparently, so did something else: dark matter.
Collectively, the entire complex of stars, gas, and dark matter is known as the Bullet Cluster.
The view of galaxies beyond those in the Bullet Cluster is "bent" by gravity. By mapping these distortions, scientists found that there's far too much gravity to be accounted for by the Bullet's visible stars and gas. The most likely source for the extra gravity is dark matter -- matter that produces no detectable energy, but that exerts a gravitational pull on the visible matter around it.
Steven Weinberg, a Nobel Prize-winning physicist at The University of Texas, explains:
WEINBERG: Most of the matter in the original two clusters of galaxies is in some form that doesn't collide with itself very efficiently -- it doesn't interact, it's not like ordinary matter. Ordinary matter -- the gas that fills the clusters of galaxies -- underwent a collision, and remained more or less in the center of the clusters, and is now glowing hot from the effects of the collision. The dark matter has just sailed through, and we see it only through its effect in bending the light, through its gravitational field, of more-distant galaxies. It's a vivid example of evidence for cold dark matter.
More about dark matter tomorrow.
Script by Damond Benningfield, Copyright 2010