This giant cluster of galaxies and hot gas, known as the Bullet Cluster, provides strong evidence of dark matter, a mysterious form of matter that produces no detectable energy but that exerts a strong gravitational influence on the visible matter around it. Two big groups of galaxies have passed through each other relatively undisturbed. Gas in the galaxies has rammed together and remained in the center of the cluster. The gas has heated to millions of degrees, producing X-rays (shown in red). The gravity of the galaxies "bends" the light of galaxies behind it. From this effect, astronomers calculate that the cluster must contain enormous amounts of dark matter (the densest concentrations of dark matter are shown in blue). Like the visible galaxies, the clouds of dark matter around them have passed through each other with no effect. [NASA/CXC/CfA/M.Markevitch; STScI/Magellan/U.Arizona/D.Clowe; ESO WFI]
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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
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