As the title says. If everything is moving outward, thrown by the Big Bang, how could two Galaxies end up with velocity vectors that run them into each other?
Answer
The universe, on a larger scale, is of course expanding, but it doesn't really mean that none of the two galaxies should be allowed to run into each other. Because expansion of the universe is not the only factor that will decide the relative separation between the galaxies. The other major factor which affect the relative separation between the galaxies is the local gravity which can essentially overcome the expansion factor for sufficiently small separation between the galaxies. Such small separation exists in reality and galaxies are evidently running into each other in these cases. The best example may be our galaxy and the Andromeda!
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