The Ontological Argument for the Existence of God

[robvalue]

Even if our timeline began at the inception of the Big Bang, there could be independent timelines which preceded it, in other realities. Something in another reality could have caused it, such as fragmentation, or this being a manifestation of a process in that reality. I see those two as the most likely explanations after "this is all there is".

But if we're just talking about our timeline, "before the Big Bang" is a nonsense term, agreed. (If we assume time began at that point.)