A galaxy is a gravitationally bound system that contains stars, stellar remnants, gas, and dust. This luminous matter is embedded in significantly larger amounts of dark matter, a telltale sign of a galaxy, as opposed to a large star cluster containing no dark matter. There are spiral galaxies such as the Milky Way, old elliptical galaxies that form from long merged galaxies, and currently merging irregular ones. The Andromeda galaxy is the Milky Way’s sister galaxy and is 2.5 million light years away. Both are gravitationally bound and on collision course. All galaxies in the universe form a giant web-like structure.
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