The universe could not suddenly exist where nothing had existed before at least not by natural means.
You are right to say that it would defy credulity to assume the universe just came about without causation. After all nothing known in the universe has come about without causation. There is always cause and effect. However, defaulting to God is not necessarily the de facto answer in the absence of explanation. Anything imaginable is possible without clear evidence to support it or refute it.
For instance, the beginning of this universe with all of its laws could have come about during the death of another universe, of which we don't know its laws.
If you'll note that the prefix in
universe means 1 (numeric), then consider the multiverse theory (many). The point is that because we don't know exactly what happened before the singularity or Planck's Time, an infinite regression takes place because of what is not known. God, though, in the absence of evidence is NOT a de facto position. You'd still have to account for evidence does favor the existence of God.
"Men never do evil so completely and cheerfully as when they do it from mistaken conviction." — Blaise Pascal
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