Most galaxies contain billions of stars, as well as gas and dust. All stars and material slowly revolve around the center of the galaxy. Galaxies form groups, while groups of galaxies form clusters. Clusters of galaxies form superclusters, between which space is mostly empty. Most galaxies are very far away from our own Milky Way galaxy; one of the closest neighboring galaxies, the Great Andromeda Galaxy (M31) is about 2.4 million light years away (the distance light travels in 2.4 million years at a speed of 300,000 km/s).
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