What does the universe expand into?

[JaceDeanLove]

I know that no one has a definitive answer to such a difficult question, but I am curious about other ideas. My personal belief, looking at physics and my (very limited)understanding of it, the universe is expanding into nonexistence. The universe, unless there's a multiverse, which is a different topic, is all that exists. In this case, all that exists is simply expanding (stretching) into nonexistence. It's hard to wrap my mind around, but it makes sense in its own way. And, disregard what I said earlier. If you think there's a multiverse, explain your belief.

Question:

Why is thinking a multiverse exists any more a belief than thinking it doesn't?

I'm sorry. I didn't mean it like that. I worded it wrong. They are both equally valid