Question

How did the Big Bang begin?

Answer 1

It was the beginning of the evolutionary epoch in the history of the Universe.

Explanation:

At Planck time (which is the earliest possible instance when time can be measured) symmetries were broken and the Universe entered the evolutionary phase (which was inflationary). Space-time became a tangible entity which we identified as the Universe. This is distinct from a singularity, which is what the primordial Universe was, before the Big Bang.