By definition, space is just the abscence of stuff but it has the capacity to have stuff put in it.
But people assume that space has to have time in it for it to be space. Our universe we occupy is fundamentally made of four dimensions, length, width, depth, and time, (and about 8 or 9 more other weird dimensions but basically four). But that's just the universe, if we could temporarily imagine the universe wasn't created and there is only nothingness then allowed for length width and depth as qualities of this nothingness, then we could put stuff in that space if there were time to allow the stuff to fill in that space. Space doesn't expand, the stuff inside of it does.
But if you ask the question you really are trying to ask, "what is length, width, and depth?" then it gets really abstract like a kid who keep asking "why?" after every answer you give him or her.
So what is length width and depth, and I don't mean the measurement of these dimensions, i mean really, what is it about our universe that allows us to move in any direction? What is a spatial dimension?