  The Milky Way's dark matter halo is expected to host numerous low-mass
subhalos with no detectable associated stellar component. Such subhalos are
invisible unless their dark matter annihilates to visible states such as
photons. One of the established methods for identifying candidate subhalos is
to search for individual unassociated gamma-ray sources with properties
consistent with the dark matter expectation. However, robustly ruling out an
astrophysical origin for any such candidate is challenging. In this work, we
present a complementary approach that harnesses information about the entire
population of subhalos---such as their spatial and mass distribution in the
Galaxy---to search for a signal of annihilating dark matter. Using simulated
data, we show that the collective emission from subhalos can imprint itself in
a unique way on the statistics of observed photons, even when individual
subhalos may be too dim to be resolved on their own. Additionally, we
demonstrate that, for the models we consider, the signal can be identified even
in the face of unresolved astrophysical point-source emission of extragalactic
and Galactic origin. This establishes a new search technique for subhalos that
is complementary to established methods, and that could have important
ramifications for gamma-ray dark matter searches using observatories such as
the Fermi Large Area Telescope and the Cherenkov Telescope Array.
