The universal vacuum cleaner

WE CANNOT see what lies beyond the visible horizon of our universe, simply because light emitted beyond that horizon has not had time to reach us. Despite this out-of-sightness, we've always assumed that space is filled with the same stuff wherever you go in the universe.

So a recent finding by Sasha Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Maryland, does not make sense. His team has found a group of galaxy clusters moving at an extraordinary speed towards a small patch of sky between the constellations of Centaurus and Vela. Kashlinsky calls it the "dark flow", in tribute to those other cosmic mysteries dark matter and dark energy (New Scientist, 24 January, p 50).