There is nothing outside the universe. The cosmic background radiation is, in a sense, the furthest thing we can observe, which is the remnant of the big bang. And yet it is the same everywhere. So the surface of the bubble actually represents a single historical point! That implies some strange geometry! The universe may seem static on a small scale (like Earth) or an expanding sphere at a larger scale (inter-galactic), but at the universal scale something even weirder is happening. It may be more useful to think of the "expansion of the universe" as an illusion created by zooming into that singularity, which is itself fixed, even unchanging, but your inexorable motion "into" it makes it look like things are flying apart. (In this case, "you" is an astronomer looking at distant galaxies; you can't really notice this effect at human-scale, and apparently you can't even measure it at lower-than-human scale, which is why quantum gravity is ...difficult shall we say.)