Galaxies traveling at million miles-per-hour speeds appear headed on one a one-way track toward the constellation Hydra, astronomers report Thursday, a "dark flow" baffling to cosmologists.

Since 1998, astronomers have known that galaxies throughout the cosmos appear to be expanding away from one another in all directions at an accelerating rate, ascribed to a "dark energy" cosmological force that counteracted gravity's attraction.

But in the Astrophysical Journal, a team led by Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md., report a group of galaxy clusters as far as 2.5 billion light years away (one light year is about 5.9 trillion miles) appear headed for Hydra, aimed in one particular direction, posing a puzzle that can't be explained by dark energy.

""This is not something we set out to find, but we cannot make it go away," Kashlinsky said, in a NASA statement. "Now we see that it persists to much greater distances," after initially detecting the "dark flow" in 2008.

To make that determination, the study team looked at the motion of galaxies against the "cosmic microwave background", a pervasive, unmoving, heat signature left over from 380,000 years after the Big Bang some 13.7 billion years ago, which has been mapped since 2001 by NASA's WMAP probe.

The stars and galaxies seen by astronomers around the one-way clusters aren't heavy enough to gravitationally pull them and explain the dark flow. "Its existence suggests that some structure beyond the visible universe -- outside our 'horizon' -- is pulling on matter in our vicinity," says the statement.

The clusters appear to be zooming along on one particular line aimed at Hydra, Kashlinsky said, but "right now our data cannot state as strongly as we'd like whether the clusters are coming or going," to or from Earth.

By Dan Vergano