Why can the universe not itself be the reason for its existence? Clearly SOME things must exist without a previous thing being there to cause them (aka god), so why not the universe?
I appreciate the fact that you have understood the need for a necessarily existing explanation. However, a necessarily existing universe has philosophical and scientific difficulties. Briefly, increasing scientific discoveries continue to confirm that the universe is not eternal but had a definite beginning, and thus cannot be a necessarily existing entity......
And? When we are talking about "explanation for existence", we are talking a How, not a Why (to be simplistic). The fact that the universe could be cyclical, or even spontaneously self-creating, both acceptably answer the How of the "reason" for its existence.
We have evidence at quantum level that particles can and do spontaneously generate. Unless your position is that those particles are (a) all gods in embryo, or (b) all specifically generated by
god (and in both cases I would need to see evidence), then we have precedent that "things just appear".
And if "things just appear", I have no issue with that being the necessary explanation of the universe, with a thing "just appearing" in nothingness and (due to the lack of constraints it would experience inside this universe) immediately starting to expand.