One of several outer space anomalies like dark energy, dark matter, and the NASA Pioneer Anomaly in our solar system. This "dark flow" anomaly is on a much larger scale.
"Dark Flow is a name given to a net motion of galaxy clusters with respect to the cosmic microwave background radiation which was found in a 2008 study. According to standard cosmological models, the motion of galaxy clusters with respect to the cosmic microwave background should be randomly distributed in all directions. However, analyzing the three-year WMAP data using the kinematic Sunyaev-Zel'dovich effect, the authors of the study found evidence of a common motion of at least 600 km/s toward a 20-degree patch of sky between the constellations of Centaurus and Vela."