Quite a few people have argued that time doesn't exist. Or at least not as we perceive it; the apparent forward flow of time only being a side effect of things like entropy. Or something. I'm not sure it makes much sense to me.

On the other hand, if you consider that time itself started (was created) at the big bang, then there was no "before" to worry about. And so, in a sense, the universe has existed forever - since the beginning of time.

But as Nereid says, this is more philosphy than science...