Galaxies are often surrounded by a halo of hydrogen gas. Over time a galaxy can lose this halo, which streams off into the intergalactic medium. As a result, there can be clouds of hydrogen among the clusters of galaxies in the universe, with a million stars worth of mass. Understanding the evolution of these hydrogen clouds can help astronomers understand how galaxies evolve. These clouds are invisible to optical telescopes, but they can be detected by radio astronomers thanks to hydrogen's 21-centimeter emission line.
