Dark matter has just gotten darker

New research published last week in the journal Science shows that dark matter interacts with itself less than previously thought. This is relevant as it could narrow down the possibilities of what dark matter actually is.

Six galaxy clusters taken with NASA/ESA Hubble Space Telescope (blue) and NASA’s Chandra X-ray Observatory (pink) (credit: NASA and ESA)

Six galaxy clusters taken with NASA/ESA Hubble Space Telescope (blue) and NASA’s Chandra X-ray Observatory (pink) (credit: NASA and ESA)

According to the standard model of cosmology there is more dark matter than ordinary matter (26.8% against 4.9%, the remaining 68.3% being dark energy), and yet dark matter is extremely elusive. It does not reflect, absorb or emit light or any electromagnetic radiation at any significant level. We know it exist based on its gravitational effects on the visible Universe.

Researchers study dark matter by investigating collisions of galaxies where clouds of gas crash into each other and slow down or stop. “We know how gas and stars react to these cosmic crashes and where they emerge from the wreckage. Comparing how dark matter behaves can help us to narrow down what it actually is” – explains David Harvey of the École Polytechnique Fédérale de Lausanne in Switzerland.

The team of scientists lead by David Harvey examined data of 72 galaxy cluster collisions from the NASA/ESA Hubble Space Telescope and the NASA’s Chandra X-ray Observatory. They found that, during these collisions, dark matter passes straight through without slowing down. This suggests that besides not interacting with ordinary particles, dark matter does not interact with other dark matter either. “A previous study had seen similar behaviour in the Bullet Cluster” – says co-author Richard Massey of Durham University, UK. “But it’s difficult to interpret what you’re seeing if you have just one example. Each collision takes hundreds of millions of years, so in a human lifetime we only get to see one freeze-frame from a single camera angle. Now that we have studied so many more collisions, we can start to piece together the full movie and better understand what is going on”.

These results narrow down the properties of dark matter and while the quest towards understanding its nature is not over, particle physicists have less unknowns to work with when building their models. Dark matter could potentially have rich and complex properties. It is possible that dark matter particles bounce off each other like billiard balls, causing dark matter to be thrown out of collisions or change shape. “There are still several viable candidates for dark matter, so the game is not over, but we are getting nearer to an answer” – concludes Harvey. “These ‘Astronomically Large’ particle colliders are finally letting us glimpse the dark world all around us but just out of reach”.

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Carlo Bradac

Carlo Bradac

Dr Carlo Bradac is a Research Fellow at the University of Technology, Sydney (UTS). He studied physics and engineering at the Polytechnic of Milan (Italy) where he achieved his Bachelor of Science (2004) and Master of Science (2006) in Engineering for Physics and Mathematics. During his employment experience, he worked as Application Engineer and Process Automation & Control Engineer. In 2012 he completed his PhD in Physics at Macquarie University, Sydney (Australia). He worked as a Postdoctoral Research Fellow at Sydney University and Macquarie University, before moving to UTS upon receiving the Chancellor Postdoctoral Research and DECRA Fellowships.

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