Gluons are force-mediating particles that exist in every atomic nucleus, holding it together. They mediate the strong nuclear force, which is the strongest of nature's four forces, 137 times stronger than electromagnetism and about 1.6 x 1039 times stronger than gravity, the weakest force. Its limitation is that it only operates on extremely small distances, the scale of the atomic nucleus. At distances longer than one femtometer (width of a medium-sized atomic nucleus) the strong force begins to fade.
The strong force holds together all known matter in the universe except for dark matter, which we know practically nothing about. So the atomic nucleus consists of a combination of nucleons (protons and neutrons) and gluons.
Like a photon (light), a gluon has no mass. It just represents a packet of force. Unlike photons however, gluons have their own "color" — the name for charge in the strong force — which means they interact with themselves, making quantum chromodynamics (strong force) more complicated mathematically than quantum electrodynamics (electromagnetism). Physicists suspect that a "glueball," an aggregation of just gluons without nucleons, might be possible, but none has yet been observed.
The gluon was first discovered in 1979 at the TASSO experiment at the Deutsches Elektronen-Synchrotron (DESY) in Germany. In typical collisions between electrons and positrons (anti-electrons) in particular accelerators, a quark and and antiquark are created, sending off two distinct particle jets which can be observed in the cloud chamber. But at sufficiently high energy, a third jet appears — which represents gluons escaping the nucleus. This provided experimental proof for the existence of gluons, whose existence had been suspected for a while.
There are eight different types of gluons total, and three different types of "color" (strong force charge). Gluons are responsible for an unusual phenomenon called "confinement." No two color-charged particles can ever be separated from each other. Unlike in electromagnetism, where the charge between two objects diminishes as they move apart from one another, the strong force remains constant and extremely powerful. Only in the most superheated and dense environments (possibly at the center of the most massive neutron stars, and in particle accelerators) do gluons and nucleons from different atomic nuclei intermesh and become what is called a quark plasma, a free-floating soup of gluons and nucleons.
