Exchange interaction independently by physicists Werner Heisenberg and Paul Dirac in 1926. The exchange interaction is sometimes called the "exchange force". However, it is not a true force and should not be confused with the exchange forces produced by the exchange of force carriers, such as the electromagnetic force produced between two electrons by the exchange of a photon, or the strong force between two quarks produced by the exchange of a gluon. Although sometimes erroneously described as a force, the exchange interaction is a purely quantum mechanical "effect" unlike other forces. Quantum mechanical particles are classified as bosons or fermions. The
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