ManWithAPlan: Well said!
In our experience in this universe we notice patterns of regularity between various events. The whole of science could be viewed as an effort to extract and formalise these regularities the underly these events.
One of these regularities, which we have uncovered through the usual inductive process is the idea of cause and effect, which orders connected events in some manner in time.
Just as we would not deem it necessary that the law of gravity holds "outside" our universe (whatever that means), it is equally fallicious to insist without argument that the Big Bang (or whatever is the true origin of the universe), necessarily has a cause.
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