Theorists really do think this, that maybe every fundamental particle has a so-far-invisible partner. The partners’ names are just the names of the regular particles only with an “s” in front: squarks, selectrons. The Large Hadron Collider (LHC) in Switzerland is kicking up a lot of dust looking for them (and for another putative particle called the Higgs); and every now and then, the LHC announces it may or may not have found something or other that undermines the foundations of the entire physics enterprise but only somewhere around a 2-sigma level which may or may not indicate these partners exist. If they do, they’ll solve Richard’s dark matter problem.
Abstruse Goose’s little jokes: particles are attractive to other particles with the opposite charge; all particles have an assigned mass; quarks (which make up the likes of protons and neutrons) come in six flavors — up, down, top, bottom, strangeness, and charm. SUSY, the name of the theory proposing the partners, would unify the four forces of the universe; it’s pronounced soosie and stands for SUperSYmmetry. SOL, I feel sure you already know.
Meanwhile, you’d do well to keep your eye on the LHC.