[quote=@Bishop]
What exists outside the universe isn't really "nothing". There are theories but still. I followed the one that says that all space has properties and contains its own energy. So space, even outside the universe isn't empty but contain dark energy. Now, no one knows the properties of this energy irl besides being the main factor in the expansion of the universe so...
[/quote]

There is no space outside of a universe. Spacetime IS the universe. If something has space, it is not outside of a universe. There is literal "nothing" outside of universes because there is no "thing" "there." "There" does not exist, and without space, there cannot be "things" "there."