  Unlike minimal SU(5), SO(10) provides a straightforward path towards gauge
coupling unification by modifying the renormalization group evolution of the
gauge couplings above some intermediate scale which may also be related to the
seesaw mechanism for neutrino masses. Unification can be achieved for several
different choices of the intermediate gauge group below the SO(10) breaking
scale. In this work, we consider in detail the possibility that SO(10)
unification may also provide a natural dark matter candidate, stability being
guaranteed by a leftover $Z_2$ symmetry. We systematically examine the possible
intermediate gauge groups which allow a non-degenerate, fermionic, Standard
Model singlet dark matter candidate while at the same time respecting gauge
coupling unification. Our analysis is done at the two-loop level. Surprisingly,
despite the richness of SO(10), we find that only two models survive the
analysis of phenomenological constraints, which include suitable neutrino
masses, proton decay, and reheating.
