If there is no God, then what (outside of time and space) caused the universe?
Imagine the question from a polytheist long ago asking an atheist: “If Thor does not exist, then what causes lightning?”
The answer the atheist would give is that he does not know what causes lightning, but he does not see his ignorance as reason to believe that the cause is Thor. He might question what Thor exactly does to cause the lightning, how does Thor cause it all over the world, what exactly is Thor and does Thor interact with other worldly phenomena, and most importantly, is there any evidence that Thor is responsible for the lightning. (Has anyone seen Thor, heard Thor, in some other way devised an experiment that supports the existence of Thor and not some other hypothesis, etc.)
Same reasoning applies to your question. I am studying physics and math, but I am still fairly ignorant about the events of the young universe. My current answer is the logical equivalent of “I don’t know” and that does not imply that the answer “God” is on any level a good hypothesis. Suppose it is possible that God exists and he somehow, someway, interacts with reality by creating universes. Would our universe be any different if there were no god but some fundamental physical law allowed for the spontaneous creation of universes? If God exists out of space and time, by default such an entity is beyond the scope of evidence, and therefore reason. In fact, it is nigh impossible to conceive of what it is “like” outside of space-time. To say “God created the universe” has exactly as much evidence and empirical meaning and significance as the explanation “nothing nothinged itself”. Is it reasonable to think that nothing nothinged itself? Should I accept it and believe it just to have an answer besides “I don’t know”? No.
So the burden is on the theist to provide 1) A definition of God and the set of its traits 2) Provide evidence or reason to believe such a deity exists 3) Give the phrase “God created the universe” explanatory power, otherwise the mystery of the universe’s existence is simply replaced with another (unexplained) mystery 4) Provide a context that proves that a universe created by a deity would differ from a universe in existence independent of a deity.
Lastly, as a side note, I’d like to point out the inconsistency of your question. At the instant the universe began its journey through time, that time was a boundary condition, implying that laws of cause and effect would not apply unlike our intuitive experience while we live in a non-boundary. If you ask what caused something that laws of cause and effect did not apply to, similarly you must address the question “what caused God?”
That pretty well sums it up. You’d think if someone really wondered about what caused the universe, they would go ask an astro physics ie google that shit. Instead they post argumentative questions on Tumblr for smart atheists to use as fodder to point out the the flaws in their logic.