The asteroid belt of the Solar System isn't of uniform density -
it has gaps at odd locations within its interplanetary domain.
Astronomer Daniel Kirkwood first documented these unexpected
vacancies in the Mars-Jupiter orbital region back in 1886.
Following is an explanation for the gaps that bear his name.
(Part of the lure of science is the discovery of something new,
and of having your name associated with some aspect of the
universe. Poor Kirkwood is now known for absences - for gaps -
for nothingness. Just how existential is that?!)  Er...ahem.
Back to the asteroids. The best explanation for their absence
involves the phenomenon of orbital resonance. An asteroid
located at the radius of one of these gaps would have an orbital
period of some whole number ratio of that of Jupiter. So every
few years (the number depending on which particular gap we're
talking about), the asteroid would be in the same place relative
to Jupiter, by far the most massive planet in the Solar System.
For most asteroids not occup