taken from wikipedia:
If the Universe had to be created by God because it must have a creator, then God, in turn would have had to be created by some other God, and so on. This attacks the premise that the Universe is the second cause, (after God, who is claimed to be the first cause). A common response to this is that God exists outside of time and hence needs no cause. However, such arguments can also be applied to the universe itself - that since time began when the universe did, it is non-sensical to talk about a state "before" the universe which could have caused it, since cause requires time.