What exactly is pulling these galaxies remains to be seen, but the NASA team believes that dark flow is actually the gravitational pull of massive objects beyond the edge of the known universe. Interestingly, the idea of matter beyond the edge of the universe doesn't violate any laws of physics. According to physicist Leonard Susskind, who was not involved with this study, the boundary of the known universe is not a wall past which nothing can exist, but simply the farthest point from which light has reached Earth. Past that, there may very well be massive objects. But since the universe is expanding, their light will never reach us, and thus we cannot observe them along the electromagnetic spectrum.