According to the Bible, God is from everlasting to everlasting. This meaning He has always existed, He never had a beginning, nor does he have an end.
Could a supreme being exist that has always existed and never had a beginning? Why not?
As finite humans, it is extremely difficult for us to comprehend no beginning and no end.
I look at is this way: Energy never dies, it converts to something else, and then to something else, and so on. Perhaps, just a theory, that God was something, than became something different, and so on and so on to the point He is at now, and is always becoming something different. Metamorphosis perhaps?