We all know that the universe is constantly in motion. Earth revolves around the sun, our solar system turns with the rest of the Milky Way galaxy, which in turn moves within a larger cluster of galaxies, and all of these components move away from other galaxy clusters as the universe expands. According to the theory of , this expansion should be equal in all directions. But by studying a map of the Cosmic Microwave Background (CMB)—the remaining radiation from the big bang that invisibly coats the universe—astrophysicists discovered that some galaxy clusters seem to be moving toward a point that appears to be beyond the perceivable universe. They call this movement dark flow.
There is a lot of debate around whether dark flow is real, and what could cause it. Several competing scientists have tried to disprove dark flow by using more detailed maps of the CMB. But as of now, it's neither fully proven nor disproven. As for what causes it, one idea is that some mass very early in the universe's existence—before cosmic inflation, when the universe was extremely compressed—made such a big impact on the matter of our universe that some galaxy clusters are still drawn to it to this day. For now, this is just a theory, but as far as astrophysics goes, it's a very exciting one.