The Great Attractor is a gravity anomaly in intergalactic space within the range of the Centaurus Supercluster that reveals the existence of a localized concentration of mass equivalent to tens of thousands of Milky Ways, observable by its effect on the motion of galaxies and their associated clusters over a region hundreds of millions of light years across.
These galaxies are all redshifted, in accordance with the Hubble Flow, indicating that they are receding relative to us and to each other, but the variations in their redshift are sufficient to reveal the existence of the anomaly. The variations in their redshifts are known as peculiar velocities, and cover a range from about +700 km/s to −700 km/s, depending on the angular deviation from the direction to the Great Attractor.
Read more about Great Attractor: Location, Debate Over Apparent Mass, The Dark Flow, In Fiction