Hypothetical particles are particles that have been theorized to exist, but have yet to be observed or proven. Quarks are the smallest known particles and are believed to make up protons and neutrons. Gluons are particles that mediate the strong nuclear force between quarks. Axions are hypothetical particles that are believed to interact with photons and could explain the matter-antimatter asymmetry in the universe. Gravitons are hypothetical particles that are believed to mediate the gravitational force.