National Aeronautics and Space Administration researchers say the cause of the motion might be the gravitational attraction of matter that lies beyond the observable universe.

"The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase," said lead researcher Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md. "We never expected to find anything like this."

Kashlinsky calls the collective motion "dark flow" in the vein of more familiar cosmological mysteries such as dark energy and dark matter. "The distribution of matter in the observed universe cannot account for this motion," he said.