Originally posted by vivify
What is causing the universe to expand?
Dark energy. In other words, energy that we have not yet identified. It is possible that empty space has a certain amount of energy, so as space expands the total amount of energy in the universe goes up, thus causing accelerated expansion.
Also, for the universe to expand, wouldn't that imply that there's something larger outside of the universe? For example, a balloon can expand if there's more space outside of the balloon to expand in; doesn't this apply to the universe?
If you were on the surface of the balloon, you would not see it as expanding into something, you would always remain on the surface of the balloon, but points on the balloon would get further apart. The analogy is not perfect because the two dimensional balloon surface sits in three dimensional space whereas three dimensional space is not necessarily expanding in a four dimensional universe.
Instead think of the cartesian plane, then try applying an enlargement to the whole plane. Everything gets bigger, and adjacent points move away from each other, but because the plane is infinite, it does not expand into somewhere else.
I don't think it is known whether or not space if finite, but it does seem likely that it is self contained.