"All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars," Aaron Robotham, a postdoctoral researcher at the International Centre for Radio Astronomy Research and head of the study, said in a statement. "Then every now and then they get completely cannibalized by some larger galaxy."
As galaxies grow, they get worse at making new stars -- but they also have stronger gravity, which helps them pull neighbors into the fold. The Milky Way reached this tipping point "recently," in cosmic terms (read: not at all recently) and will now grow mostly by snacking on the little guys. It's been a while since our neighborhood ate another one, but astronomers can still see the signs of former galaxies that we've digested.
But The Milky Way isn't going to be able to outrun Andromeda. In about 5 billion years we'll collide with the nearby galaxy, which contains at least twice as many stars as our own. To Andromeda, we'll be nothing but a cosmic candy bar.
These cannibalistic mergers will continue until the whole universe is made of just a few gigantic galaxies, but that's a long way off -- a destiny we won't reach until the Universe is many times older than it is today.