(I can't comment)- See https://physics.stackexchange.com/questions/136860/did-the-big-bang-happen-at-a-point
TLDR, The big bang happened everywhere in the universe at the same time, because it was the universe. The top answer goes more in depth.
This paper gives a possible explanation as to the missing antimatter, it suggests matter collides with antimatter in such a way the resultant ejected particles are real on-mass shell particles
Antimatter and the Big Bang, J.C.O'Brien
So according to general relativity, there are events, in space-time. There are many different ways of putting coordinates on these events so that each one can be said to occur at a particular (x,y,z,t) set of coordinates and the laws of physics all work correctly when you put in those xs, ys, zs and ts, even though many intuitive questions like "did ...