  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 favoured 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
$\sim 10^{12}$ GeV, and discuss their astrophysical observability.
