We were once taught that the universe meant everything there is. Then we learned that the universe is expanding. Expanding into what? If there's room for it to expand, what's in that room now? Gizmodo asked various scientists what is at the edge of the universe. Most of the experts started out by explaining that over the distances we'd need to find out, we can only see into the past because of the time it takes for light to reach us. That's the "observable universe." Caltech physics professor Sean Carroll goes on to explain,
Because we can only see so far, we’re not sure what things are like beyond our observable universe. The universe we do see is fairly uniform on large scales, and maybe that continues literally forever. Alternatively, the universe could wrap around like a (three-dimensional version of a) sphere or torus. If that were true, the universe would be finite in total size, but still wouldn’t have an edge, just like a circle doesn’t have a beginning or ending.
It’s also possible that the universe isn’t uniform past what we can see, and conditions are wildly different from place to place. That possibility is the cosmological multiverse. We don’t know if there is a multiverse in this sense, but since we can’t actually see one way or another, it’s wise to keep an open mind.