Okay, here's my $.02 on the matter, but, please notice my moniker!
I really do not subscribe to the "Big Bang" theory, but if we are to believe that the whole universe started from one little tiny point in the middle of absolutely NOTHING ( a Singularity ? ), then the answer would have to be , EVERYWHERE !! I "grog" this by reasoning that if NOTHING suddenly becomes EVERYTHING, then there would be no CENTER or STARTING POINT because there would not have been a goegraphical or 3-dimensional "foci" of said "singularity" PRIOR to the Theorized event!!
If we are to truly believe that the Universe indeed has a "starting point", and that it was indeed some 14 billion years in the past - how can we, as a civilization hope to believe that we have "completely figured the full workings of the whole universe" after only a few thousand years of "cognitive" thinking!!
Look, i've already admitted to being the dumbest man on earth, so there is a full possibility that I am, once again, wrong when I do not believe that there is indeed any way to "know it all"!!
So, to sum it up; If, notice the if, IF the "Big Bang" happened, then the answer to - Where did it happen? - would have to be......HERE !!!!
Later, the dumbest man on earth