There's no notion of spacetime outside the universe. The spacetime is the universe. Two assumptions: a) the universe is infinite. In this case it doesn't expa

There's no notion of spacetime outside the universe. The spacetime is the universe. Two assumptions: a) the universe is infinite. In this case it doesn't expand into anything, the things inside of it just stretch apart. b) the universe is finite. In this case it expands into something, but because we're trapped inside spacetime, we can't answer this scientifically. The wording "expanding" adds more confusion than it helps. See [1] for the discussion about this topic. [1]: http://curious.astro.cornell.edu/ask-a-question/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/623-what-is-the-universe-expanding-into-intermediate http://curious.astro.cornell.edu/ask-a-question/104-the-univ...