My question is "what is at the end of the universe if it really does end?" because if the big bang theory is correct then then when the whole universe was one atom or very small what did it expand into because if there was nothing there it couldnt have expanded. for example if you have a room and you say that the walls are the end of the universe then you build on to the room to make it bigger (the universe expanding) there has to be room on the other side of the wall for you to build into.
There is nothing called the end of the Universe. There are three possibilities of the shape of the Universe.
First, the Universe might have what we call positive curvature like a sphere. In this case, the Universe is called "closed" and it has a finite size but without a boundary, just like a baloon. In a closed Universe, you could, in principle, fly a spaceship far enough in one direction and get back to where you started from.
The second possibility is that the Universe is flat. This kind of Universe can be imagined by cutting out a piece of a baloon material and stretching it with your hands. The surface of the material is flat and not curved. You can expand and contract it by tugging on either end. Flat Universes are infinite in extent and have no boundaries.
Finally, the Universe might be "open" or have negative curvature. Such Universes are also infinite in spatial extent and have no boundaries.
Thus whatever be the shape of the Universe, there is nothing called a boundary and hence nothing called the edge or end of the Universe.
Regarding the second question of expansion, remember that space exists only IN the Universe and there is no meaning to the term "outside the Universe". What happens in expansion is that the space itself is expanding. With respect to your room analogy, it is not that the walls of your room are pushing against something but that the space in the room is expanding; there is nothing to push against. Thus, when we talk of galaxies receding from us due to the expanion, it is not that the galaxies are moving, but the space in between us and the galaxies is expanding.
Edit by Michael Lam on February 10, 2016: Recent observations suggest that the Universe is very close to flat. The WMAP mission determined that it is flat to within a 0.4% margin of error.