Expanding Universe

Overall, the universe is expanding, and any two galaxies you choose will, on average, be moving apart as it does. Locally, though, gravitational attraction between galaxies can overcome the general expansion, creatiing "peculiar motions" that are superimposed on the expanding universe. Peculiar motions can make galaxies separate more slowly than they otherwise would, or even overcome the expansion altogether-- the Milky Way and the Andromeda galaxy, for example, will have a close encounter, even collide, in ten billion years or so. Cluster of galaxies, more massive than single galaxies, exert even stronger gravity and create larger peculiar motions. The Milky Way and Andromeda, besides approaching each other, are together being pulled in the direction of the great cluster of galaxies in the constellation Virgo.