[evman150]

Notwithstanding theories about "multiverses", which are at this point untestable, all that exists is the universe.

So saying "What lies beyond our expanding universe?" is a little like saying "What lies outside the baseball stadium inside the baseball stadium?". Or "what is the largest integer from 1-10 that is not 1-10?".

We are dealing with a complete set, actually THE complete set, outside of which nothing exists, only because we define the set as everything. Something existing outside of everything is nonsense.

What is it expanding into? Itself is the best answer. There is no medium it is stretching in. Space itself is stretching, so to speak. We never hit anything, there is no "nothingness" or otherwise. Just space stretching.

The north pole example does not obtain. If you're going to say "what is above X", the location of X is arbitrary. You're shifting the parameters of your thought experiment in an ad hoc manner. You can't go more north than the north pole. "Up" is not north.