Because they are composed of quarks, baryons participate in the strong interaction, which is mediated by particles known as gluons. The most familiar baryons are protons and neutrons, both of which contain three quarks, and for this reason they are sometimes called "triquarks". These particles make up most of the mass of the visible matter in the universe and compose the nucleus of every atom. (Electrons, the other major component of the atom, are members of a different family of particles called leptons; leptons do not interact via the strong force.) Exotic baryons containing five quarks, called pentaquarks, have also been discovered and studied.