'In ancient Greece, Democritus put forth the idea that solid objects were comprised of atoms of that element or material, either jammed tightly together, as in the case of a solid object, or separated by a void (space). These atoms were thought to be little indivisible billiard-ball-like objects made of some sort of “stuff.” Thinking this through a bit, it was apparent that if atoms were thought to be spherical and they were crammed together in an optimal fashion, then matter was essentially 74% of the space that it takes up, the rest being air, or empty space. So, for example, a solid bar of gold was really only 74% gold “stuff,” at most.
'That view of matter was resurrected by John Dalton in the early 1800s and revised once J. J. Thompson discovered electrons. At that point, atoms were thought to look like plum pudding, with electrons embedded in the proton pudding. Still, the density of “stuff” didn’t change, at least until the early 1900s when Ernest Rutherford determined that atoms were actually composed of a tiny dense nucleus and a shell of electrons. Further measurements revealed that these subatomic particles (protons, electrons, and later, neutrons) were actually very tiny compared to the overall atom and, in fact, most of the atom was empty space. That model, coupled with a realization that atoms in a solid actually had to have some distance between them, completely changed our view on how dense matter was. It turned out that in our gold bar only 1 part in 10E15 was “stuff.”
'That was, until the mid-60’s, when quark theory was proposed, which said that protons and neutrons were actually comprised of three quarks each. As the theory (aka QCD) is now fairly accepted and some measurement estimates have been made of quark sizes, one can calculate that since quarks are between a thousand and a million times smaller than the subatomic particles that they make up, matter is now 10E9 to 10E18 times more tenuous than previously thought. Hence our gold bar is now only about 1 part in 10E30 (give or take a few orders of magnitude) “stuff” and the rest in empty space. By way of comparison, about 1.3E32 grains of sand would fit inside the earth. So matter is roughly as dense with “stuff” as one grain of sand is to our entire planet.' (The Universe - Solved! weblog).