This is what we call inconceivable. It's a concept which, with the definitions by context that humans are subject to, we cannot define, nor understand, because there's nothing to compare it to other than itself.

As far as I'm concerned, the universe is in an infinite cycle of expansion/compression. It is one of the few theories that is widely criticized, yet accepted by the scientific community, in that "What happened to bring that single atom to such unimaginable energy that it exploded in what was the Big Bang, creating the Universe?" The simple answer is... Well, the universe was there. As our universe "expands" it has limits to which it may expand, in that there's only so much mass it can press out with the remaining energy.
So, what's left when it stops expanding? The tremendous force has to give, in the most simple of rules of "Equal and opposite reaction." Our universe is quite possibly just a reactionary force of something more, but what that is, it's anyones guess.
But what will end up happening, is the Universe will condense in on itself in the same way it expanded, reverting to it's area of a single atom. All the force of that compression, all that mass forced into such an infinitesimally small area will create what can only be described as the most powerful force in the Universe: All kinetic energy focused onto a single point, the Universe itself. Thus, another explosion, another Big Bang, another reformation of a new form of the Universe.

One key thing to notice about the universe is... Everything from the formation of atoms to the formation of solar systems, galaxies, it's all in a revolutionary force, the pieces revolving around a central point. The universe's formation is exactly that of an atom, and just as an atom can be split and smashed and torn apart, so too can it be compressed into itself, no different than a collapsing star.

Edit: Geekygirl, one thing you have to keep in mind is that God, this paternal figure, wonderment, freedom, intelligence, everything that you ever think you know is nothing but creation of man, they're just words and concepts. The actual things themselves are inconceivable. In a sense, humans are nothing but self-indulgent egomaniacs.