Ask Ethan: Where did the Big Bang happen?
Ethan Siegel

Agree with Vinson Ryan Nash.

You could also put it like this: If Big Bang started in a very high density state, then even if you imagine that state as something very small, point-like, it was still the entire universe, with no ‘outside’ of that state. So when that high density state started to expand rapidly, it was the entire universe that expanded, and consequently the expansion started everywhere.