The origin of the universe is explicable - you may not like the explanation and therefore disagree with it, but that is not the same as saying it can't be explained. "Since the universe did not cause itself into existence" is an assumption. Working from that, your logic appears sound - but that does not mean any god has to be the default designer. He's a candidate, but not the only one. But all that's irrelevant seeing as you base your conclusions on an unwarranted assumption.