Hidden-sector fermions in two or three generations
  We recall the argument based both on Dirac's square root procedure and an intrinsic Pauli principle that sterile fundamental fermions with spin 1/2 (sterinos) ought to appear in Nature in two or three generations, while the Standard Model leptons and quarks are forced to develop three generations. Then, sterinos in two or three generations, if stable, lead to the cold dark matter in two or three species, when the thermal freeze-out mechanism works properly. If they are stable only in their lowest generation, the so called exciting cold dark matter may be formed. 
Wojciech Krolikowski