Universe Tugged by Mysterious 'Dark Flow'

Sept. 25, 2008 -- Astronomers have stumbled upon an unexplained two-million-mile-per-hour sideways shift in the universe toward a colossal, unseen, unknown gravity source beyond the horizon of the observable universe.

What's being called a dark flow appears to be pulling vast clusters of galaxies toward a 20-degree-wide patch of sky between the constellations of Centaurus and Vela.

"It does fly in the face of everything we know," said astronomer Dale Kocevski of the University of California at Davis. He's one of the authors of a paper in the Sept. 24 issue of Astrophysical Journal Letters which introduced the discovery. "I'm sure it's going to be controversial."

The dark flow was detected by studying 700 very distant clusters of galaxies which are lit up by hot, X-ray-emitting gases.

WATCH VIDEO: Learn about the moment astronomers found the universe was expanding.

First the team of researchers led by NASA's Alexander Kashlinsky carefully located the X-ray clusters -- each containing thousands of galaxies.

Next, they looked at the same spots on a map of what's called the cosmic microwave background -- the attenuated glow from the first light that was free to travel through space just 380,000 years after the universe was born. This glow was mapped in detail by NASA's Wilkinson Microwave Anisotropy Probe (WMAP).

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According to theory, when the ancient microwaves pass through galaxy clusters they should change temperature in predictable ways, depending on whether the galaxy is moving relative to the background glow. So this work started as an experiment to test that effect -- what's called the kinematic Sunyaev-Zel'dovich (SV) effect -- and to see if any movement could be detected.

"We were hoping to measure something there, but probably not much," said Kashlinsky. "To our great surprise what we found instead is that the velocity was quite higher than expected."