  Dark matter in the halos surrounding galaxy groups and clusters can
annihilate to high-energy photons. Recent advancements in the construction of
galaxy group catalogs provide many thousands of potential extragalactic targets
for dark matter. In this paper, we outline a procedure to infer the dark matter
signal associated with a given galaxy group. Applying this procedure to a
catalog of sources, one can create a full-sky map of the brightest
extragalactic dark matter targets in the nearby Universe ($z\lesssim 0.03$),
supplementing sources of dark matter annihilation from within the Local Group.
As with searches for dark matter in dwarf galaxies, these extragalactic targets
can be stacked together to enhance the signals associated with dark matter. We
validate this procedure on mock $\textit{Fermi}$ gamma-ray data sets using a
galaxy catalog constructed from the $\texttt{DarkSky}$ $N$-body cosmological
simulation and demonstrate that the limits are robust, at $\mathcal{O}(1)$
levels, to systematic uncertainties on halo mass and concentration. We also
quantify other sources of systematic uncertainty arising from the analysis and
modeling assumptions. Our results suggest that a stacking analysis using galaxy
group catalogs provides a powerful opportunity to discover extragalactic dark
matter and complements existing studies of Milky Way dwarf galaxies.
