If it did, then it is an effect of something (of God, or the Big Bang etc)
That logic is simply wrong on its face. Causality, as you lay it out, is a consequence of time. But time is a physical property of the universe and exists within it. The universe does not exist in time.
Once again, the north-south coordinate has a definite beginning and a definite ending, but this doesn't imply that anything lies south of the south pole. At the south pole, all possible directions point north, including directions that lie at right angles to each other. Similarly, at the Big Bang, in the simplest (Friedmann-Robertson-Walker) model, all possible directions point towards the future. If there's geometrically no past, there can't be a prior cause.