We’ve known since 1922 that the universe has been changing since it came into being, but we’re still not sure about all the details. A better word might be stretching, rather than expanding. If the cosmos is infinite, then it’s always been infinite and it will stretch forever; it’s not expanding into anything.
If it’s finite, it may be stretching out something else, but we can only postulate. Currently astronomers believe the universe is either infinite or somehow folds back on itself.
Answered by Shanna Freeman