  We present evidence of the gravitational lensing of the cosmic microwave
background by $10^{13}$ solar mass dark matter halos. Lensing convergence maps
from the Atacama Cosmology Telescope Polarimeter (ACTPol) are stacked at the
positions of around 12,000 optically-selected CMASS galaxies from the
SDSS-III/BOSS survey. The mean lensing signal is consistent with simulated dark
matter halo profiles, and is favored over a null signal at 3.2 sigma
significance. This result demonstrates the potential of microwave background
lensing to probe the dark matter distribution in galaxy group and galaxy
cluster halos.
