Why shouldn't the original singularity of the big bang happen to be the centre of the Universe? Assume that the universe is expanding isotropically with a constant speed.
(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.