The eternal inflation model says that there's an infinite number of universes, with new universes constantly popping into existence and expanding outwards. Every so often, these new universes might collide. Our universe has four "bruises" possibly caused by these collisions.
We've already found possible evidence of a universe before the Big Bang in the cosmic microwave background, so why not other universes? This particular theory comes out of a model known as eternal inflation, which suggests universes are constantly popping up and expanding throughout the vast, unimaginably infinite cosmos.