[RightWhale]

The universe is mostly empty space

Empty of some things, packed to the gills with other things. No part of the universe is absolutely empty. If nothing else there is that three degree microwave radiation everywhere.

[doc30]

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.

[Doctor Stochastic]

If nothing else there is that three degree microwave radiation everywhere.

It's full of gravity, too.