  Within the context of $N=1$ supersymmetric heterotic $M$-theory, we present a
"freeze-in" mechanism for producing dark matter via a "moduli portal" between
the observable and hidden sectors. It is assumed that the observable sector
consists of the MSSM or some physically acceptable extension of it, while the
hidden sector is chosen so as to satisfy all physical and mathematical
constraints. Dark matter production processes are examined for two fundamental
types of hidden sectors; those whose gauge bundle structure group contains an
anomalous $U(1)$ and those whose structure group is non-Abelian and anomaly
free. The couplings of the dilaton and the "universal" modulus to all fields of
the observable and hidden sectors are presented and analyzed. These
interactions are then combined to produce a moduli portal from a thermal bath
of observable sector particles to the hidden sector. These processes are then
analyzed for both the anomalous and non-anomalous cases. It is shown that only
the uncharged hidden sector matter scalars can play the role of dark matter,
and that these are predominantly produced during the "reheating" epoch on the
observable sector. Within the context of both an anomalous and non-anomalous
hidden sector, we calculated the dark matter "relic density". We show that in
both case, for a wide choice of moduli vacua, one can correctly predict the
observabled relic density. For the anomalous $U(1)$ case, we choose a specific
physically acceptable vacuum within the context of the $B-L$ MSSM and show that
one precisely obtains the measured dark matter relic abundance.
