[oblomovshchina]

Nothing. The concept seems foreign because we're always in something, but there is definitionally nothing the Universe can expand into. If it's easier to conceptualize, just think of 'expansion' as a short way of saying 'distances are getting longer'.

[fareesh]

So does it mean that everything in the Universe is getting bigger but since everything is getting bigger we don't know it because relative sizes are the same?