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Who said that the universe started from nothing?
You appear to be confusing our current inability to observe our universe and the current insufficiences of scientific theories to explain what happened before the big-bang, as being evidence for nothing being in existence before the big bang.
You can theorize (belief) that the universe was eternal without requiring a belief in a deity.
Time is a measure of decay. If nothing is decaying, there is, at the most fundamental level, "no time." We have a terrible time coming to terms with a "timeless" event because our existence is steeped in time, hence our trouble understanding a concept like "before time."
This is essentially the Kalam cosmological argument (the idea that every event has a cause, thus there must be a first cause from outside our universe, thus, God). It doesn't hold up.
I like the Klein bottle analogy when confronted by this. It helps stop my brain from melting.
It is illogical to suggest that time had a cause, as "cause" is an event that preceded another event in time. So suggesting that time itself had a cause is nonsensical.
Everything at the macroscale, in time, and moving at a speed less than light is subject to causality, that is what our observations suggest. However, inferring that the universe had a cause because events within the universe have causes is a fallacy of composition. It is fallacious to infer things about a whole based on its individual parts.
Also, what does it mean that god was the "uncaused cause" if you have just excluded "something coming from nothing", and "the universe having always existed" as possibilities? You can't exclude both, either the matter and energy that made up the universe always existed in some form or another, or it came into exist from nothing at some point. You can't exclude both, there is no third option.
If god "caused" the universe to pop into existence from nothing, then that is still an example of something coming from nothing whether it was caused or not. If god merely fabricated the universe from pre-existing material, then the universe always existed is some form or another.
As for "why is there something rather than nothing", it begs the question. It assumes without justification that "nothing" is the natural and original state of affairs of existence, and "something" therefore requires explanation -- this is an unjustified assumption, there is no reason to suppose that, so the question does not require an answer, it is a non-problem created by an unjustified presupposition.