  Individual extragalactic dark matter halos, such as those associated with
nearby galaxies and galaxy clusters, are promising targets for searches for
gamma-rays from dark matter annihilation. We review the predictions for the
annihilation flux from individual halos, focusing on the effect of current
uncertainties in the concentration-mass relation and the contribution from halo
substructure, and also estimating the intrinsic halo-to-halo scatter expected.
After careful consideration of recent simulation results, we conclude that the
concentrations of the smallest halos, while well determined at high redshift,
are still uncertain by a factor of 4-6 when extrapolated to low redshift. This
in turn produces up to two orders of magnitude uncertainty in the predicted
annihilation flux for any halo mass above this scale. Substructure evolution,
the small-scale cutoff to the power spectrum, cosmology, and baryonic effects
all introduce smaller, though cumulative, uncertainties. We then consider
intrinsic variations from halo to halo. These arise from variations in
concentration and substructure, leading to a scatter of $\sim$ 2.5 in the
predicted annihilation luminosity. Finally, we consider the problem of
detecting gamma-rays from annihilation, given the expected contributions from
other sources. We estimate the signal-to-noise ratio for gamma-ray detection as
a function of halo mass, assuming that cosmic rays from star formation are the
main noise source in the detection. This calculation suggests that group-scale
halos, individually or in stacks, may be a particularly interesting target for
the next generation of annihilation searches.
