proposed concentration of mass that influences the movement of many galaxies, including the Milky Way (Milky Way Galaxy). In 1986 a group of astronomers observing the motions of the Milky Way and neighbouring galaxies noted that the galaxies were moving toward the Hydra-Centaurus superclusters in the southern sky with velocities significantly different from those predicted by the expansion of the universe in accordance with the Hubble law (see Hubble's constant). One possible explanation for this perturbation in the Hubble flow is the existence of the so-called Great Attractor—a region or structure of huge mass (equivalent to tens of thousands of galaxies) exerting a gravitational pull on the surrounding galaxies. It is estimated that the Great Attractor would have a diameter of about 300 million light-years and that its centre would lie about 147 million light-years away from Earth.