IOW, what started the Big Bang? Basic cause & effect.
"basic" cause and effect?
What you refer to by "basic" is actually just classical newtonian-style physics as it applies within a space-time continuum.
These physics fall apart both at the quantum level as well as at T = 0 (= the start of the space-time continuum, aka the universe).
There is no phenomenon of causality as we know it at the quantum level.
And as causes happen
before effects, causality necessarily requires temporal conditions. ie, the existence of a time-dimension. A dimension which is not present at T = 0.
To ask about "what happened 'before' the universe" thus becomes asking "what happened 'before' time" which is a non-sensical question.
It's like asking "who lives north of the north pole". There is no north of the north pole....