The "bullet cluster" 1E0657-56 is a cluster of galaxies that has two clumps. These two clumps recently collided setting up shock waves. The bow shock on the right in the red image is especially prominent. These shock waves slowed down the hot gas in the clusters, while the dark matter and galaxies continued traveling rapidly to the left and right. The blue image shows the total mass density measured using gravitational lensing. The source of gravity is clearly not centered on the hot gas, but is centered on the galaxy clumps. But the hot gas weighs about 10 times more than the galaxies. So there clearly has to be another unseen mass that weighs much more than the hot gas: this is the cold dark matter or CDM. The CDM does not emit light, nor does it interact with matter or with itself, because if it did, the CDM would have slowed down in the shocks and stayed with the hot gas.
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© 2006 Edward L. Wright. Last modified 22 Aug 2006