Paul Sutter in Ars Technica:
Beginning as early as the 1970s, astronomers noticed something funny going on with the galaxies in our nearby patch of the Universe. There was the usual and expected Hubble flow, the general recession of galaxies driven by the overall expansion of the Universe. But there seemed to be some vague directionality on top of that, as if all of the galaxies near us were also heading toward the same focal point.
Astronomers debated whether this was a real effect or some artifact of Malmquist bias, the bias we get in our observations because bright galaxies are easier to observe than dim ones (for fans of statistics, it’s just another expression of a selection effect). It could be that a complete census of the nearby cosmos, including the much more numerous small and dim galaxies, would erase any apparent extra movement and return some sanity to the world.
But then came more detailed observations of the cosmic microwave background (CMB).