Well, that *was* a theory for a very long time; Once the Big Bang was discovered, it was thought that the gravity of all the mass from all the galaxies in the universe *must* be slowing the expansion. The big question for years was, will it stop? Or expand forever?
25-30 years ago, when we finally were able to measure the rate of expansion, it was a surprise to most that it was, in fact, *speeding up.* Today, it looks like the expansion not only won't stop, but will keep accelerating. "The end of everything" is looking more and more like it might be "The Big Rip."