Galactic Cluster Collision Divides Ordinary Matter from Dark MatterSAlmost six billion years ago, two of the largest gravitational structures in the universe slammed into each other with velocities in the millions of miles per hour. Galactic clusters are collections of galaxies (sometimes thousands of them) that seem to hang together in violation of the known laws of physics. Not only did this ancient collision result in a stunningly beautiful image, but it's given astrophysicists an important clue about the nature of dark matter.The image above is a composite, combining data from the Hubble Space Telescope with x-ray imagery from the Chandra X-Ray Observatory. Hubble captured the blue areas by detecting the gravitational lensing of light shining around the cluster. Dark matter in the cluster is causing the lensing. The pink area is a mass of hot gas, made of ordinary matter, which radiates the x-rays detected by Chandra. What's amazing about the image is how clearly it shows that the dark matter separated from the ordinary matter when the clusters collided. The gases interacted with each other gravitationally, causing them to slow down and "pile up" in the middle of the clusters. The dark matter evidently did not interact with itself, sliding to the outer edges of the clusters. Astronomers think the mass of all this dark matter is what holds clusters together - without it, the galaxies are moving too fast to stay together. The cluster collision provides a lot of direct evidence that dark matter exists. Image by: NASA/ESA. You can see the full image in high-resolution here. Collision of galaxy clusters captured by astronomers. [EurkeAlert!]