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Dark flow is a perceived anomaly in the gravitation of several galactic clusters, centering between the constellations of Centaurus and Vela. While the motions of galaxies are believed to be more or less randomly distributed with respect to the cosmic microwave background, a peculiar force seems to be pulling a particular cluster in the same direction at similar speeds. This anomaly cannot be accounted for by the general expansion of the Universe and it appears as if something outside and beyond our Universe is pulling these clusters together.
While more recent data from the Planck spacecraft seem to discredit this (such as this study, that shows an Universe expanding at the same velocity elsewhere), the authors of this hypothesis claim these same data support its existence. Check the Wiki article and references therein.