  In grand unified theories with gauge groups larger than SU(5), the multiplets
that contain the known quarks and leptons also contain fermions that are
singlets under the Standard Model gauge group. Some of these could be the dark
matter of the universe. Grand unified theories can also have accidental U(1)
global symmetries (analogous to B-L in minimal SU(5)) that can stabilize dark
matter. These ideas are illustrated in an SU(6) model.
