Baryon

From Slackerpedia Galactica

A baryon is any subatomic particle composed of three quarks. Protons and neutrons are familiar baryons, out of which most of the matter in the universe that we actually see is made. They're called "nucleons", because you can make atomic nuclei out of them. Other kinds of baryons are named after squiggly greek letters, and are primarily of interest to physicists.

We haven't the faintest idea why most of the universe isn't made up of baryons since there seems to be quite a bit of gravitating stuff floating around.