  We study dark matter (DM) which is cosmologically long-lived because of
standard model (SM) symmetries. In these models an approximate stabilizing
symmetry emerges accidentally, in analogy with baryon and lepton number in the
renormalizable SM. Adopting an effective theory approach, we classify DM models
according to representations of $SU(3)_C\times SU(2)_L\times U(1)_Y \times
U(1)_B\times U(1)_L$, allowing for all operators permitted by symmetry, with
weak scale DM and a cutoff at or below the Planck scale. We identify
representations containing a neutral long-lived state, thus excluding dimension
four and five operators that mediate dangerously prompt DM decay into SM
particles. The DM relic abundance is obtained via thermal freeze-out or, since
effectively stable DM often carries baryon or lepton number, asymmetry sharing
through the very operators that induce eventual DM decay. We also incorporate
baryon and lepton number violation with a spurion that parameterizes hard
breaking by arbitrary units. However, since proton stability precludes certain
spurions, a residual symmetry persists, maintaining the cosmological stability
of certain DM representations. Finally, we survey the phenomenology of
effectively stable DM as manifested in probes of direct detection, indirect
detection, and proton decay.
