There's a question the physicist Enrico Fermi supposedly blurted out over lunch in 1950, somewhere between the soup and the punchline of a joke about flying saucers. The galaxy is old — around 13 billion years. It's enormous — hundreds of billions of stars, a large fraction with planets. Even at sluggish, sub-light speeds, a single expansionist civilization could colonize the whole thing in a few tens of millions of years. That's a rounding error on cosmic timescales.