How Dark matter Interacts with Ordinary Matter

New measurements of a group of colliding galaxies appear to indicate that the mysterious substance likely interacts with itself and ordinary matter only via gravity. This reversed many conclusions scientists had drawn from observations three years earlier.

About 27 percent of the universe is dark matter, but scientists still know very little about what it is in real. This dark matter doesn’t emit or reflect light, which makes it hard to study. Its gravity can bend the path of light in phenomenon known as gravitational lensing, which has helped astronomers to ascertain that something is out there.

[source Chandra X Ray Laboratory ]
A team of researchers used Hubble Telescope, 3 years ago, to observe galaxies colliding in the Abell 3827 cluster, located about 1.3 billion light-years from the Earth. The galaxies’ dark matter seemed offset from the visible matter in the collision, which the scientists said could indicate that dark matter might feel other forces in addition to gravity.

Abell 3827
This image shows the four central galaxies in the Abell 3827 cluster with ultraviolet light from the Hubble images (blue) and infrared light from the ALMA observations (red). At these wavelengths, scientists can determine the extent to which a galaxy behind the cluster has been distorted by the gravity of the normal and dark matter. [source]
They revisited the observations for a new study in the Atacama Large Millimeter/ submillimeter Array (ALMA) in Chile. This is a power telescope which was able to pick out details that Hubble observation didn’t caught. The data revealed the location of previously undetected dark matter around the collision.

“We got a higher resolution view of the distant galaxy using ALMA than from even the Hubble Space Telescope,” Liliya Williams, researcher at the University of Minnesota and co-author of this work. “The true position of the dark matter became clearer than in our previous observations.”

This indicates that most of the galaxies’ dark matter stayed with them during the collision. The dark matter either exclusively feels the effect of gravity or that it interacts only weakly via other forces. Alternatively, the cluster could be moving towards Earth, in which case we wouldn’t expect to see any sideways displacement in the dark matter, the scientists said in the statement.

Assuming this to be true the dark matter would have shifted either in front of or behind the cluster, making the offset hard to detect. Worldwide astronomers are looking into the sky for clues about the nature of dark matter. Many hypothesis have evolved to explain the substance as scientists use computer models to get a better idea of what to look for. “Different properties of dark matter do leave tell-tale signs,” Andrew Roberston, researcher at Durham University in the UK and co-author of this work.

“One especially interesting test it that dark matter interactions would make clumps of dark matter more spherical,” Roberston.”That’s the next thing we’re going to look for.”

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