The definition of "universe" is something like: All of space, all of time, and everything that occupies them. So anything that can be named or described is, by definition,
… in the universe. There's nothing like Truman crashing his sailboat into the wall at the boundary of everything he had ever known. A different view: The best way to answer this is to describe what is in it. All Matter, space, and time are in it. So therefore outside of it is no matter, no space, or time. Btw since the universe is expanding, such places where there is no space, matter, or time, do exist. Another Perspective: It is generally believed that there is no "outside" to the universe. The universe is everything in existence. It has no boundary, i.e. there is no point at which all the matter of the universe is behind you with an endless void ahead of you. It is likely that the universe in infinite, but even if it is not, that does not necessarily mean it has a boundary. The universe may be "curved", meaning that if you go in one direction for long enough you end up back where you started (as is the case on the surface of the earth). (MORE) 
