While you know very little about the origin of the universe, astronomers know a lot more. However, since the universe is by definition everything, it did not originate by an external cause. It came to be as an uncaused event. There is, by definition, no such thing as "before the universe came to be."
Well, "know" is a very strong word. We have a theory, it fits certain facts and has problems with other ones. In my cosmology course last term our instructor guaranteed us that most of what he was teaching us was probably wrong- the point of the class was to learn how to evaluate the current evidence and the evidence that will be produced in the future. He always said, "Would I lie to you?" (WILTY) when telling us a possible lie.
Some physicist once said, "Cosmologists are always wrong and never unsure."
It isn't really relevant to the main thrust of the discussion, since I think we're making acceptable progress toward a description of the early universe, but there are still many unanswered questions. It is meaningless to speculate about "before the big bang", certainly from a scientific point of few, even if it came out of the collapse of a prior universe there is fundamentally no way to recover this information (WILTY?).
Philosophically however, the argument is not as neat as you would make it. To insist that everything has a cause today, and then define the first event as having no cause... well, it must be true of course, but on the other hand, how can it be? If the universe could "just become" without cause (as it must have), then why can things today not "just become" without cause too? Perhaps they can.
This is why God still has a place in things. You can postulate God as some sort of special different thing, not bound by causation like matter- you can postulate that a God can "just be" only once, eliminating the worry about other universes springing into "just being" without cause.
Answering the question "by definition" normally raises lots of other questions.