They are not seen so the masses of the , must be . Explains the relationship between quarks and leptons. Sub-structure scale deviations not seen. Excited quarks and leptons not seen. There is no evidence for compositness and it is not seen as a serious contender these days.
Every boson has a supersymmetric fermion partner.
This means that supersymmetric particles can only be produced in pairs from normal particles. The lightest supersymmetric particle must be stable. A heavy supersymmetric particle can decay to a lighter one The lightest supersymmetric particle cannot decay to another supersymmetric particle and cannot decay to ordinary particles. We do not know which is the lightest. We assume that the Universe is full of them left over from the big Bang.