Whoa! Thatís quite a question! And the answer actually lies on the border between science, philosophy, and stuff we just donít know. But rather than answering it myself, let me give you a (rather long) quote from Dr. Leon Lederman, a very cool (and quite famous) particle physicist:
"In the Very Beginning there was a void -- a curious form of vacuum -- a nothingness containing no space, no time, no matter, no light, no sound. Yet the laws of nature were in place, and this curious vacuum held potential. Like a giant boulder perched at the edge of a towering cliff . . .
Wait a minute.
Before the boulder falls, I should explain that I really donít know what Iím talking about. A story logically begins at the beginning. But this story is about the universe, and unfortunately there are no data
for the Very Beginning. None, zero. We donít know anything about the universe until it reaches the mature age of a billionth of a trillionth of a second -- that is, some very short time after creation in the Big Bang. When you read or hear anything about the birth of the universe, someone is making it up. We are in the realm of philosophy. Only God knows what happened at the Very Beginning (and so far She hasnít let on).
Now, where were we? Oh yes . . .
Like a giant boulder perched at the edge of a towering cliff, the voidís balance was so exquisite that only whim was needed to produce a change, a change that created the universe. And it happened. The nothingness exploded. In this initial incandescence, space and time were created.
Out of this energy, matter emerged -- a dense plasma of particles that dissolved into radiation and back to matter. (Now weíre working with at least a few facts and some speculative theory in hand.) Particles collided and gave birth to new particles. Space and time boiled and foamed as black holes formed and dissolved. What a scene!"
(from The God Particle, copyright 1993 by Dr. Leon Lederman and Dick Teresi, published by the Houghton Mifflin Company
He goes on to describe the universeís growth and expansion, but the important part for your question is right here. Basically what heís saying is that the universe exists because it can
exist. Before the universe, there was nothing. But there was the chance
for that nothing to become the universe. And since the possibility existed, it was bound to happen eventually. And so here we are.
[There are other possible pictures suggested since that was written, as mentioned below.]
(published on 10/22/2007)