The centers of rich clusters of galaxies contain the densest concentrations of matter in the universe. They’re also among the most violent places we know of. As time rolls on and large galaxies swarm around meeker ones, mergers take place. Big galaxies grow larger by eating small ones. As this happens, worlds are torn apart, stars shredded, and gas clouds compressed into reckless new throes of star formation. We live in a relatively quiet corner of the Milky Way Galaxy. By contrast, the centers of rich galaxy clusters are the universe's most chaotic locations, constantly bustling with activity.

Until recently, astronomers thought they understood how galaxy clusters form. As matter collapses inward, pulled by gravity, groups of galaxies and clumps of matter crush together. The monsters of the scene, the big galaxies, fall toward the center, where the most mass resides.

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Hot gas in the cluster core loses energy and cools by emitting X-rays. As the gas inside the cluster cools, it also contracts. Astronomers dubbed this contracting gas a cooling flow. Up until 2006, the idea had been gospel since first proposed in 1977.