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ALMA Observations Show Dark Matter Does Not Interact With Itself

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The dark matter, which makes up about 85 percent of the matter in the universe is known to interact with ordinary and visible matter only through gravity.

Abell 3827

About three years ago, however, a team of scientists made observations that led them to conclude that dark matter may interact with other forces other than gravity.

Cosmologist Richard Massey and colleagues used an instrument on the European Southern Observatory's Very Large Telescope, and images from the Hubble Space Telescope to observe Abell 3827, a cluster of four colliding galaxies 1.3 billion light-years away.

They found that a clump of dark matter appears to move more slowly than the host galaxy, a behavior that would happen if dark matter was interacting with another clump of dark matter through other forces besides gravity.

Massey and colleagues thus deduced hat dark matter can interact with another clump of dark matter, which contradicts the standard theory.

"We used to think that dark matter just sits around, minding its own business, except for its gravitational pull," the researchers said. "But if dark matter were being slowed down during this collision, it could be the first evidence for rich physics in the dark sector - the hidden Universe all around us."

ALMA Finds Signs Of Previously Undetected Dark Matter

New observations by the same team, however, now rule out this kind of interaction. In their new study, which will be published in the journal Monthly Notices of the Royal Astronomical Society, Massey, from Durham University in England, and colleagues used observation made with the Atacama Large Millimeter Array or ALMA in Chile and found that dark matter was still behaving exactly as expected.

By using ALMA, the researchers found signs of dark matter that they were not able to detect in the previous study. They found that the fourth galaxy has not actually separated from its clump of dark matter.

"We got a higher resolution view of the distant galaxy using ALMA than from even the Hubble Space Telescope," study researcher Liliya Williams, from the University of Minnesota, said. "The true position of the dark matter became clearer than in our previous observations."

The researchers said that the new observation means that dark matter still behaves as it should and it still interacts only through gravity.

"We looked for longer and found the dark matter was hiding just where it ought to be," Massey said. "It's a sort of eating humble pie on some level."

Massey and colleagues said that conclusions can change when data improves and they will continue to study dark matter. Despite the frustrating new findings, they said that the hunt continues to unveil the mysteries of dark matter.

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