Whatever God is, the fact that the universe exists proves that God exists.
If only that was so - we wouldn't have 400+ pages in this thread.
We have no evidence of anything that can make the complexity and size of the universe. Whatever could do the making, would fit the general description and definition of God. If it didn't, it wouldn't be able to make something like the universe with all its "furnishings."
''We have no evidence of anything that can make the complexity and size of the universe.'' You are right badecker we don't, why do you claim you do then?
As you say, we don't have evidence of any thing that can make the complexity of the universe. That leaves room for God, only. God, not being a thing, or anything, has made the universe.
What proof do we have for empty space, which is essentially nothing at all? Does it really exist? Yes. We can tell that "nothing" exists by measuring the things that DO exist, and their relationships. In a similar way, even though we cannot directly "lay hands on God" to "measure" Him and prove Him thereby, we can prove He exists by measuring the complexity of the universe, especially with regard to cause and effect.