That's my problem. There must be "something" into which to expand. So what's it expanding into? If it's expanding into a "void", then the void itself is something. In your analogy, what is the balloon floating in? I'm starting to think it's a parallel universe and we are encroaching into that space.
According to the Big Bang theory, there is absolutely nothing outside the expanding universe. All that is is being created within the expansion. Furthermore, there is no one center to the expansion. Everywhere is the center. ie, every point within the universe see everything else expanding away from it at the same (accelerating) rate.