I pretty much explained my views on this matter in the thread
"Rationalistic Theism"
Personally I don't believe we can ever know the answer. We certainly don't right now. I think that the multidimensional universe as Dragonlich stated in #4 is quite probable. However it still fails to address the ultimate question: where did THAT come from?
Cause and effect would dictate that it had to be CAUSED by something, but you must remember that such concepts as "time" and something happening "before somethign else" are completely confined within our own universe.