  Since the gravitational interaction is universal, any particle that ever
existed, if kinematically accessible, has to be produced through her. We
explore the possibility that dark matter is generated purely from gravitational
scatterings together with heavy Majorana right-handed neutrinos that are
long-lived. Their late decay could inject significant entropy into the thermal
bath, diluting both the dark matter abundance and the cosmic baryon asymmetry,
thereby imposing various constraints on the reheating dynamics. Additionally to
the entropy injection, long-lived right-handed neutrinos could also be
responsible for generating the baryon asymmetry through leptogenesis, and hence
establish some nontrivial relations between the dark matter and the
right-handed properties, and the reheating dynamics
