Actually, I was referring to matter in space. It is mostly empty, but as you correctly pointed out, there is a lot of matter concentrated in a few places (e.g. neutron stars). But my comment still stands. Even what we call solid objects, like my desk or your car, are mostly empty on the atomic scale. There is still a lot of room for compression.
Even such a simple thing as an electron is not a solid particle with a given dimension. It cannot be said that there is empty space between a proton and an electron in a simple hydrogen atom since for all we know the electron fills the entire space. We cannot say what the diameter of a proton might be except by specifying a process of bouncing another proton off it and measuring the bounce. What most call solid matter is not solid matter but could well be nothing at all but some multidimensional curvature of space. If nothing else, the curvature exists and if that is what solid matter is then space is filled completely.