Dark matter is indistinguishable from dark energy.
 At an even deeper level, protons and neutrons are made up of quarks and the force fields (gluons) that bind them together, leading to the next definition
 Force mediators are usually not considered matter: the mediators of the electric force (photons) possess energy (see Planck relation) and the mediators of the weak force (W and Z bosons) have mass, but neither are considered matter either

