Argument from Contingency
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I understand that you think the universe needs an explanation, but why couldn't the universe itself simply be the necessary thing? Why introduce God at all?
Because the universe appears to contain changing and contingent states. If its existence depends on conditions that could have been otherwise, it doesn't seem to contain the sufficient reason for its own existence.
But aren't you assuming that every contingent thing needs an explanation? Maybe reality simply bottoms out in a brute fact.
Calling something a brute fact doesn't explain it. The question is whether contingent reality can terminate the explanatory chain when its own state could have been otherwise.
Then what would actually make an explanation ultimate? At some point, don't you still have to stop asking why?