Quantum chromodynamics possess an additional SU(3) gauge degree of freedom, later called color charge. Han and Nambu noted that quarks might interact via an octet of vector gauge bosons: the gluons. Since free quark searches consistently failed to turn up any evidence for the new particles, and because an elementary particle back then was "defined" as a particle which could be separated and isolated, Gell-Mann often said that quarks were merely convenient mathematical constructs, not real particles. The meaning of this statement was usually clear in context: He meant quarks are confined, but he also was implying that the strong interactions could
Elementary particle one can conclude that most of the visible mass of the universe consists of protons and neutrons, which, like all baryons, in turn consist of up quarks and down quarks. Some estimates imply that there are roughly baryons (almost entirely protons and neutrons) in the observable universe. The number of protons in the observable universe is called the Eddington number. In terms of number of particles, some estimates imply that nearly all the matter, excluding dark matter, occurs in neutrinos, and that roughly elementary particles of matter exist in the visible universe, mostly neutrinos. Other estimates imply that roughly elementary