Exactly, that's the question! It's amazing to me that the answer to so mundane a question as that could be so important for cosmologists. Yet apparently, it could tell us lots of new things about matter, dark matter, and the nature of the universe itself. It seems like THAT in itself ought to be telling us something.
There is no general agreement of what space is, or time, or matter. In physics class we had to answer up quite quickly and brightly of course. They don't allow the philosphy dept in the same building as the physics and engineering dept for good reason.