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Giant filament of dark matter connects galaxy clusters, say astronomers (+video)

For the first time, astronomers say they spotted a string of dark matter via gravitational lensing. 

By Clara MoskowitzSpace.com / July 5, 2012

A July 2012 study of the galaxy clusters Abell 222 and Abell 223 found they are connected by a dark matter filament, shown here. The blue shading and the yellow contours indicate the density of matter. The image on the sky is about twice as big as the full moon.

Jörg Dietrich, University of Michigan/University Observatory Munich.

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A giant string of invisible dark matter has been discovered across the universe between a pair of galaxy clusters.

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Scientists have further narrowed the search for a hypothetical particle that could be dark matter, the mysterious stuff that makes up 80 percent of all the mass in the universe. This video from NASA Astrophysics presents the new results, compiled from two years' worth of data from NASA's Fermi Gamma-ray Space Telescope.

The filament forms a bridge between two huge clusters called Abell 222 and Abell 223, which lie 2.7 billion light-years away. The universe is thought to be filled with such strings of dark matter, a mysterious substance that cannot be seen, only sensed through its gravitational pull.

Scientists have made previous attempts to find dark matter filaments, which are predicted by theories that suggest galaxy clusters form at the intersections of filaments. Dark matter is thought to make up 83 percent of all matter in the universe.

"This is the first time [a dark matter filament] has been convincingly detected from its gravitational lensing effect," said astronomer Jörg Dietrich of the University Observatory Munich, in Germany. "It's a resounding confirmation of the standard theory of structure formation of the universe. And it's a confirmation people didn't think was possible at this point."

Many astronomers thought detecting filaments would have to wait until telescopes became significantly more advanced, but Dietrich and his colleagues benefited from the rare spatial geometry of this cluster, which allowed them to detect signs of what's called weak gravitational lensing. [Gallery: Dark Matter Throughout the Universe]

According to Albert Einstein's general theory of relativity, massive objects warp space and time around them, causing anything traveling through it, including light, to proceed along a curved path.

When light from a background object, such as a galaxy, passes nearby a massive cluster on its way to Earth, its image will appear bent because of the crooked path its light traveled.

"The standard wisdom is that the gravitational lensing of filaments is too weak to be detected with current telescopes," Dietrich told SPACE.com. "Only when we realized this system has such a peculiar geometry did we realize we have a chance."

Abell 222 and Abell 223 are arranged so that they appear very close together against the sky, but are farther apart along our line of sight looking away from Earth. This means that most of the mass of the system is condensed into one small area of the sky that any light arriving at Earth from behind will have to pass though, boosting the gravitational lensing signal.

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