Well, actually, we really have no idea what caused the Big Bang, but it definitely wasn't the motion of subatomic particles! All we really know is that the universe is currently expanding, and our observations of the universe imply that this expansion started with some kind of incredibly strong explosion. But the important thing to keep in mind is that it's not the matter in the universe that's expanding today - it's space itself. That is, it's not the case that before the Big Bang, the universe was empty, and then all of a sudden something went BANG and created lots of matter. As far as we know, the Big Bang was the beginning of the universe itself - space, time, and matter. So if time itself began with the Big Bang, it really doesn't make sense to ask what happened before it!

Of course, the question is then to understand what caused the bang in the first place, and this is what we have very little understanding of. Some theories say that perhaps our universe is only one of many, and that a "collision" between our universe and another caused the Big Bang. Some theories say that perhaps it's not quite right that spacetime began at the Big Bang, but that there was a universe that existed before ours which collapsed and then "bounced" back out again to turn into our universe, and this bounce is what we see as the Big Bang.

Unfortunately, it's really hard to test any of these theories, because it's really hard to make observations that far back in time!

The beginning of our universe is one of the least-understood (and most interesting!) questions in physics today, so I can't really give you many definitive answers. But you're asking the right kinds of questions!

The big bang theory is a theory that assumes that the whole universe, all energy and matter in it, started as a condensed, enormously hot and dense, point like state 13.7 billion years ago. That means that at this point, all the stars, galaxies, planets, everything was condensed into one single point.

From then on, after the "big bang" which can be thought of as a gigantic explosion, the universe has been expanding, with the galaxies that came into existence moving away from each other. Evidence for this fact has been observed and is one of the experiments supporting the big bang theory, as opposed to a theory of the universe which assumes that the universe has simply always been there.

Another important piece of evidence in favor of the big bang theory is the cosmic microwave background radiation, which is not so easy to explain, but may be thought of as a remainder of the flash of light coinciding with the "explosion" of the big bang. The continuous expansion of the universe is what allows us to still "see" this remainder.

So the result of the big bang would be the coming into existence of the universe as we know it, not just our galaxy but all others, as well. What exactly happened DURING the big bang is obviously not easy to investigate, and more a matter of theory than experiment. This is easy to understand if you liken the big bang to an explosion - it's hard to say what exactly happened at the point of the explosion right as it was happening, or rather to relate what was happening to something familiar.