The multiverse spacetime paradox? If there are indeed multiple universes, where do they exist? We "know" that outside our universe, consisted of spacetime, there is nothing - i.e. 0 dimensions, no space and no time. So "where" are other universes, how would the distance between our space and other be measured inside nothing?
Read Edwin Abbot's book Flatland.

In this story we visit a vision of a two dimensional "Universe" embedded in a higher dimensional space. Now by analogy we may suppose that our Universe is embedded in a higher dimension space with plenty of room for other (3+1)-Universes.

I do not claim this is a viable model of "reality" just a thought experiment to show "where" other universes might be if there are indeed multiple universes. Also Flatland is a fun read.