If the answer to the question 'Who/what is God?' would be 'The entity that created the universe and everything in it', from a scientific viewpoint, the mass of energy that started the big bang would be it*. (Unless there's an omnipotent being behind that, ofcourse ...)

The follow-up question ofcourse would then be: 'Is God sentient'.

But in the end, if 'he' is/was sentient or not doesn't really matter, cause it's not something you can really prove through scientific means.

* Certainly would give new meaning to the term: God is all around us :P

Just thinking out loud here ...