@Captain Spod
I can see where you're coming from and have probably thought along similar lines myself. I could be mistaken but think the common conception is of the big bang being something which occurred 'somewhere in space', that is a space which already existed.
The tricky concept to grasp is there was no space. A Sci-Fi writer might refer to it as "un-space" but scientists would regard that as a meaningless term.
Anyway, picture an X,Y,Z three-dimensional graph, where the axes start at zero size (the singularity) and shoot outwards, as the big bang occurs. Coordinates 0,0,0 must exist but knowing our own coordinates, relative to that point is not straightforward.
I've seen computer-animated 3-D maps of the structure of the known universe. The distribution of matter is uneven and does show gigantic voids, giving an overall structure like froth. For the record, our galaxy is nowhere near the centre.
From our knowledge of everyday things, explosions blast outwards and any void left behind is instantly refilled by air. For me, the expansion of the universe is more like expanding foam - thin films of matter (galaxies) around expanding voids.
As for *why* space expands, I haven't a clue. I still haven't encountered the explanation for 'acceleration' in the rate of expansion. (Red shifts have been studied for close to a century, so how fast can the rate of change be?)