I am not convinced that the universe had an origin. (As AlexK points out, the current model of a big bang does not take us back all the way to a beginning, so you cannot use that to 'prove' an origin, even if it is exactly correct as far as it goes.) If the universe did have an origin, I do not know what it was. And neither does anyone else. But I do know that saying "god did it" is useless twaddle that explains nothing.
One of the speculative ideas that I rather like is the idea that in the future, everything will be drawn back together by gravity and then will all come crashing together, and ricochet off in all directions. Perhaps that is what happened in the past, and is the cause of the 'big bang.' Of course, this is speculative, and may well be false. The simple fact is, we do not know what will happen in the future either. (I seem to recall reading something that the current ideas are suggesting that this particular speculative idea is wrong, but that will only be known one way or another after a few more things are figured out. AlexK can tell us what the latest is on such ideas, if he wishes.)
Additionally (and stop reading AlexK!),
One of the speculative ideas that I rather like is the idea that in the future, everything will be drawn back together by gravity and then will all come crashing together, and ricochet off in all directions. Perhaps that is what happened in the past, and is the cause of the 'big bang.' Of course, this is speculative, and may well be false. The simple fact is, we do not know what will happen in the future either. (I seem to recall reading something that the current ideas are suggesting that this particular speculative idea is wrong, but that will only be known one way or another after a few more things are figured out. AlexK can tell us what the latest is on such ideas, if he wishes.)
Additionally (and stop reading AlexK!),
"A wise man ... proportions his belief to the evidence."
— David Hume, An Enquiry Concerning Human Understanding, Section X, Part I.
