Astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize in the last century that most of the matter in the universe was not visible. About 84 percent of the matter in the cosmos is dark matter, much of it located in halos around galaxies. It was dubbed dark matter because it does not emit light, but it is also mysterious: it is not composed of atoms or their usual constituents like electrons and protons.
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