The universe is everything that is. Nothing inside the universe explains itself, so the universe as a whole can’t explain itself. Therefore God is necessary, to explain the universe.

But if God explains the universe, what explains God? If you say that God is completely self-sufficient, and doesn’t need an explanation, why can’t I say the same about the universe? If you posit a Meta-God that explains God, you set off an infinite regress: your Meta-God now needs explaining. If you say “fine, I accept an infinite hierarchy of Meta-Gods”, the question still recurs: what explains the infinite hierarchy as a whole?

God really explains nothing at all. Which is the opposite of God explaining the universe.