It's an interesting question indeed.

Because saying that the universe is expanding INTO something would be illogical as that "something" is already a part of that very universe. The universe doesn't have any rigid boundaries. It includes everything there is.

It "expanding" might be the result of something called dark energy which is a theory first proposed by Einstein. In simple words, it the opposing force of gravity. While gravity pulls in, dark energy pushes out. So in theory, if gravity were the dominant force in our universe, the universe would contract.

Correct me if I'm wrong.