It's really difficult (and I mean really difficult) to conceptualize, but the "wavefront" of universe's expansion may represent the finite-but-increasing limit of space itself.

Beyond that? "Beyond" would be a null and meaningless concept if this is the case. "Beyond" would be nothing...not vacuum, but literally nothing...non-existence.

As for multiverse theory, I personally consider it unlikely. I understand what's being asserted reasonably well (at least for a math-adept layperson), but I'm not convinced it's the most likely theory.