BS, if I told you that I had a tiny purple hippo living in my pocket would you believe me? Would you have any doubts? Could you state with certainty that I did not? Why is this god business any different?
Is it not presumptuous to say that because we don't understand something a big invisible old man must exist, or an entity, or whatever? Isn't it less presumptuous to say we don't really know but we have a couple of theories going? Isn't it less presumptuous to strive for an answer rather than to say it was the all-loving murderer in the sky?