That's actually wrong. For one thing, there is no such thing as a true vacuum. That's more or less what quantum physics is all about, but even if a "true vacuum" what you have in there is space. Space is a thing unto itself. The theory of early universal spatial expansion is there to describe and explain the fact that, given current data on the age of the universe and its apparent size, it has to have, at some point, expanded faster than the speed of light. The problem here is that nothing can "move" faster than light (see: relativity), however space can EXPAND faster than the speed of light.
This is a useful analogy that is wrong in almost ever fundamental sense. Imagine that all matter, planets, stars, etc, is suspended in a three dimensional grid at fixed points. The grid is space, if you expand the area between each point on the grid the things suspended within, for all intent and purposes, move apart, however that are not, technically speaking, moving at all. This allows for the size of the universe and the distribution of the matter therein to be what it seems to be without violating general or special relativity.
The "Light curtain" some of you seem to be referring to is actually just what is known as the "Observable Universe". This is the bubble of universe from which light will eventually reach Earth. Everything beyond this bubble will never reach us because of the accelerating expansion of the universe.
What defines the interior of the universe is space and physical law. There is no "outside" that we could comprehend, because it's not even nothing, it's way beyond what we would consider nothing, and even if there was something, say a border universe we could hop over to, you'd probably instantly die because the laws of physics there would almost certainly be different, and we depend on those laws every second of the day, which there might not even exist.