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Dark matter can clump into cigar shapes

Elusive dark matter around clusters of galaxies often clumps into cigar shapes, new observations show.
Image: Dark-matter image
A Subaru Suprime-Cam image shows one of the clusters used in the dark-matter analysis. The purple hue shows dark-matter distribution measured by the gravitational lensing effect on distant galaxies, with darker colors indicating denser concentrations. The image shows that the dark-matter distribution is elongated along the northwest-southeast direction.Subaru Telescope
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Elusive dark matter around clusters of galaxies often clumps into cigar shapes, new observations show.

The discovery could help scientists finally understand what makes up dark matter, which is the mystifying stuff thought to exist invisibly all around us. Dark matter, which could be more than five times more abundant than visible matter, is only detectable through its gravitational pull on regular material.

According to the new observations, the dark matter around many galaxy clusters is a flattened, cigarlike shape, rather than a rounded sphere.

"There are clear theoretical predictions that we expect dark matter haloes to be flattened like this," said study co-author Graham P. Smith of the University of Birmingham in Britain. "It's a very beautiful, very clean and direct measurement of that."

Smith and the team, led by Masamune Oguri of the National Astronomical Observatory of Japan and Masahiro Takada at the University of Tokyo, used a quirk of gravity called gravitational lensing to observe dark matter's gravitational effects on large collections of galaxies known as galaxy clusters. Gravitational lensing occurs when mass warps space-time, causing light to travel along a curved path when it passes by. The amount of curving can tell astronomers how massive celestial objects are.

For this study, the researchers used the Prime Focus Camera on the Subaru Telescope on Mauna Kea in Hawaii to observe 20 galaxy clusters. They took advantage of gravitational lensing to create maps of the distribution of mass around the clusters, thus getting a peek into the secrets of dark matter.

"What we're probing with these gravitational lensing observations is the dark matter distribution, because the dark matter dominates the mass on these large scales," Smith told

The fact that the dark matter seems to be flattened out into oblong shapes fits in with the so-called cold dark matter theory. Computer simulations based on this theory have predicted such shapes, but they have never before been verified to such an extent with so many large clusters.

The findings could shed light on the fundamental nature of this weird stuff, which scientists cannot detect directly. The observations support the possibility that dark matter is actually made of tiny particles called WIMPS (weakly-interacting massive particles) that exert a strong gravitational force, but otherwise don't interact with normal matter.

The research will be detailed in the Monthly Notices of the Royal Astronomical Society.