|MadSci Network: Astronomy|
You've made your mistake in your opening sentence: the Universe was made all at once, at least on the scale you're thinking about. The expansion of the Universe doesn't relate to a continual (that is, extended in time) creation of more space; it comes about because, in our understanding of the physics of the Universe, the Universe isn't stable. That is, it must either be expanding or contracting, unless there's some unknown physical process operating to counterbalance its natural predisposition to expand or contract. (In the cosmology business, this "unknown physical process" is called the "cosmological constant" or "capital lambda".) This has been in the news recently (late 1998/early 1999), and you can read a little about it here, and more about cosmology in general here; the latter is presently kept very up-to-date but may be at too deep a level for you.
The observable Universe is something else; this is that part of the Universe which we can see, because there has been time enough for light (moving at a speed of one light-year per year) to reach us from those places. This is approximately everything within the distance (age of the Universe) times (speed of light), that is, 15 billion light-years or so. More of it is coming into our view all the time, because as time goes the light from on more and more of it can reach us. Those parts have existed for as long as the rest of the Universe, but they haven't previously been visible. That the Universe has a definite age, and light has a fixed speed, means that there always is a part of the Universe that is just now becoming visible to us.
Try the links in the MadSci Library for more information on Astronomy.