Nothing.

The 4D Hyperspace with other universes in it.

"The great unknown"

that's a kind of border-limit question; if there was something out of the universe wouldn't still be part of the universe? if not what would distinguish something out of the universe from something in the universe, wouldn't just our conception of border expand?

beyond the physical universe is an infinitely big spiritual realm.

also, I believe in a multiverse, but I don't know if the multiverse is a place within the spiritual realm or what.

Even more sparse nothingness of space probably. It's hard to think about the border itself, let alone beyond that.

I don't know. It's kind of like trying to imagine the colours that aren't visible to the human eye.

More universe ?

Figment of our imaginations.

probably some kind of multiverse that contains a lot of other universes

All the socks that disappear from our dryers