[Cboldt]

The notion of "solid," at least as we perceive it, derives from the Pauli exclusion principle. Electrons are observed to obey this principle, no two may be in the same state/space. We naturally develop a sense of reality that no two objects can share the same space, at the same time, e.g., no passing through walls without leaving a trace.

BUT, not all particles are bound by this principle. I forget which particles are free to stack up on one another in the same spot - neutrinos maybe?

I'm a bit fond of teaching my kids that the closer scientists look at matter, the more it disappears.

The mechanical range of scale of at least intellectual inquiry is mind boggling, from the Planck length (which I think is on the order of a trillionth of the "width" of a proton or something like that) to billions of light years - and as you point out, nearly all of the volume of the universe, even the so-called "solid" stuff, is empty.

[BenLurkin]

A Capella Science - Bohemian Gravity
http://www.youtube.com/watch?v=2rjbtsX7twc

[Boogieman]

“I forget which particles are free to stack up on one another in the same spot - neutrinos maybe?”

Any boson particle, so photons, bosons, gluons, etc. Fermions, like electrons, quarks, and neutrinos cannot share states.