  Using high-resolution microwave sky maps made by the Atacama Cosmology
Telescope, we for the first time present strong evidence for motions of galaxy
clusters and groups via microwave background temperature distortions due to the
kinematic Sunyaev-Zel'dovich effect. Galaxy clusters are identified by their
constituent luminous galaxies observed by the Baryon Oscillation Spectroscopic
Survey, part of the Sloan Digital Sky Survey III. We measure the mean pairwise
momentum of clusters, with a probability of the signal being due to random
errors of 0.002, and the signal is consistent with the growth of cosmic
structure in the standard model of cosmology.
