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Almost 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. More »
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