Natural candidates for superheavy dark matter in string and M theory

We reconsider superheavy dark matter candidates in string and M theory, in view of the possibility that inflation might generate superheavy particles with an abundance close to that required for a near-critical Universe. We argue that cryptons—stable or metastable bound states of matter in the hidden sector—are favored over other possible candidates in string or M theory, such as the Kaluza-Klein states associated with extra dimensions. We exhibit a specific string model that predicts cryptons as hidden-sector bound states weighing $\sim {10}^{12}\hspace{0.5em}\mathrm{GeV},$ and discuss their astrophysical observability.