It is a principle.. a law that exists inside the universe.. let me call it a universal law or principle (I do not mean general by universal.. i only intend it to be universal as it exists inside the universe). When we make inferences using our universal laws or principles, we are bounded by the limitation of the word "universal".
Our knowledge of the world about us comes from us perceiving them using our senses and we induce the existence of such principles and we confidently generalize it to everything around us.
If there was something outside the universe (super natural realm or whatever) we can have Zero knowledge about it as we do not have the abilities to experience it at all. and so, we can never claim that our universal laws and/or principles apply in that supposed realm that is beyond our experience.
Hence, you premise should be more like:
1) Everything that exists inside the universe has a cause (derived from the universal principle of causality).
2) The universe exists
Therefore, I am unable to conclude if the universe itself follows the law/principle that is within it and therefore, the universe does not necessarily have a cause. Maybe it does, maybe it doesn't.. there is no way to find out.
That is unless we start experiencing the "other bigger prior universe or realm"(assuming it exists) and find out that the rules within our universe (that we know exists and induce our understanding of its principles) are the same rules in that other secret hidden universe/realm.
Not mentioning that if the rules were the same and we did experience that hidden realm, we would ask the same questions regarding that hidden universe and following your same reasoning we would think there is a one prior to it "more hidden" and agnostics over there would tell you my argument lol... unless we see god in person in it

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I hope this gets through...
In all honesty, its far more complicated than that.... my basic point is that you cant use universal principles to talk about things "outside" of it.