What came before the Big Bang?
There was no Big Bang as a first moment, and there was no before. Before is an ordering of recorded events, set by which record read which, and where nothing has been recorded the word has nothing to order. The hot, dense era about fourteen billion years ago is real and well evidenced, and its evidence is records held today: the microwave glow, the ratios of the light elements, the spectra of galaxies, the catalogs of supernovae. The Big Bang names the place where that trail of records goes dark. Winding the expansion back to a first moment needs a single clock running for the whole universe, and no observer holds one. The universe as a whole is a timeless history caused by the loop it closes. Observers inside it build the screen that computes it, in their subjective future, so nothing switches it on from an earlier moment or from outside.
Also in Chapter 35: What Is Anything Made Of?, Chapter 37: Why Does the Loop Have No Beginning?.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read appendix a: Eighteen Questionswatch a universe make its own particles