Astronomers have a problem, galaxies are not moving in the sky the way they should. There is a mysterious object known as the "great attractor" which is termed a "gravity anomaly" as it is the origin of a large amount of gravitational force. Unfortunately, this gravitational anomaly is hiding behind the Milky Way, so we cannot really observe it too well as our own galaxy obscures it. Dark Flow is a related concept, only much larger than the great attractor. There is some mysterious force, at the edge of the visible universe, that is sending entire galaxies hurtling along at nearly 3.2 million kilometers per hour. The great attractor has a known influence of at least fifty thousand Milky Ways, but the Dark Flow anomaly is greater. Dark Flow is gravitational influence from outside the observable universe.
for more information:
http://www.nasa.gov/centers/goddard/news/releases/2010/10-023.html
http://en.wikipedia.org/wiki/Dark_flow
