I don’t think it’s possible to say. Our current definitions of spacetime is limited to what is in the observable universe. What’s on the other side could be a whole different universe with similar spacetime. Or it could be an infinite void. Or it could literally not exist. Or it could be a vast sea of cream cheese. We don’t know, and according to our current understanding of physics, it is impossible for us to ever know.

Edit: not a physicist by the way, would also love someone who actually knows what they’re talking about to butt in.

Most physicists would view this as pointless debate. Since it doesn't make predictions, it's not really even a theory. It's not even wrong.

I believe Penrose and Hawking both like this idea, or rather the idea that black holes spawned "baby universes" [1].

Something is falsifiable if you can think up some empirical test that would refute it. That test doesn't need to be feasible. In the case of black hole universes, such a theory may be falsifiable, but we don't know enough about the composition of black holes to even formulate the tests. I'm not a physicist, but my impression is that falsifiability is less important to physics than the ability for a hypothesis to predict and explain observed phenomena.

[1] https://www.salon.com/2020/10/18/why-physics-nobelist-roger-...

You're probably joking, but in case not, I would point out that black holes evaporate quickly at small scales, and cannot be smaller than the Planck length.

One of my pet peeves is this vague idea about the universe being an infinite fractal, universes inside atoms, etc. It's an appealing idea, but it's totally inconsistent with quantum mechanics and the difference between the strength of forces on different length scales.