Unless we figure out a way to take measurements of areas outside of space-time to compare against (which seems quite impossible), it’s probably a question humanity will never answer.
It might be a question with no meaning: The universe interacts with nothing else. It spawned out of nothing, and its expansion is only meaningful if you’re inside the universe to see it happening.
It could be a question with a lot of meaning: perhaps the universe exists on top of some higher dimensional substrate that is conducive to big-bang style expanding universes. Maybe the reason the universe expands is only possible to answer by having access to the information of what it is expanding into.
It could be question impossible to wrap our heads around: Maybe the area outside the universe runs on metaphor, and our universe expands in a sense that would make more sense to a writer than a physicist.
Basically, endless sci-fi can be written about that question. But given we are (probably) restricted to staying within our universe’s laws of physics, it’s quite likely we’ll never really know.
