This optical image shows a cluster of galaxies. The X-ray image of the same cluster shows a colossal cloud of gas that does not appear in the optical image. The cloud is 5 million light years across and contains enough matter to make hundreds of trillions of stars. The cloud is thought to have been heated by gravitational collapse when the universe was one half of its present size. Is this cloud part of the "missing matter" problem? Will Chandra find similar clouds surrounding other galaxy clusters?

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Slide 18 of 27