Science, in the year 2013, doesn't know yet. But that's not a good enough reason to conclude it was God.
The most obvious next question which must follow the assertion that it was a God which created the Universe is "What created God?".
Common responses to this question by theists involve the prospect that this is an unanswerable question, that the question makes no sense due to the fact that by it's very nature, God is eternal, that it has been around forever and doesn't need a creator.
But why bother taking this extra step? Why don't the theists conclude that the origin of the universe in an unanswerable question, that humans will never know. Or why not conclude that the universe is eternal, that is has always been here. Maybe we are in just one universe, which is part of a multiverse which has been here forever; why not conclude this?
Just because you don't know, or even science doesn't know yet, this is not justification for claiming that a god caused it. From a lack of knowledge, it does not logically follow to the conclusion that it was a god. It's that simple.