Dark matter: the plot thickens

Mysterious substance that doesn't interact with light doesn't interact with itself, either

Six of the galaxy clusters studied by Hubble and Chandra. Regions with lots of gas (as seen in X-ray emissions) are coloured pink/purple while the distribution of dark matter (as revealed by its gravitational lensing effects) is shown in blue.
Credit: NASA/ESA/STScI/CXC/D. Harvey/R. Massey/T. Kitching/A. Taylor/E. Tittley

By Russell Deeks

Scientists using the Hubble Space Telescope and the Chandra X-Ray Observatory to study collisions between galaxy clusters have concluded that the mysterious substance known as dark matter is even 'darker' than was previously thought: not only do dark matter particles not react with ordinary matter or light, they don't interact with each other, either. But while the results may seem discouraging for those attempting to pin down what the invisible substance is made of, they actually help to eliminate some contender particles from the search.

Dark matter is believed to make up nearly 27 per cent of the mass of the Universe, over five times that made up of ordinary matter (that is, everything from stars and planets to you and the screen you're reading this on), yet we know very little about it, because - as stated above - it cannot be seen and doesn't interact with anything around it. To get a better handle on how it does behave, scientists have been using Chandra and Hubble to study galaxy cluster collisions.

When galaxies (and, by extension, galaxy clusters) collide, different elements of their make-up behave differently. Clouds of gas and dust bump into each other and slow down or stop, while stars themselves largely breeze on through unperturbed, because they are so far apart. After studying 72 different collisions between galaxy clusters, a team led by David Harvey at the École Polytechnique Fedérale de Lausanne in Switzerland found that dark matter, too, remains largely unaffected when such collisions occur.

"We know how gas and galaxies react to these cosmic crashes and where they emerge from the wreckage," says Havey. "Comparing how dark matter behaves can help us to narrow down what it actually is."

As dark matter is thought to be evenly distributed throughout regions where it is present, the team concluded that dark matter particles must not interact with each other in a way that creates a strong frictional force (which would cause dark matter particles, like dust or gas, to slow down). However, other forms of interaction - such as if particles bounce off each other like billiard balls, or if particles change shape as a result of colliding with others of their kind - have yet to be ruled out. It is hoped that similar studies of individual galaxy collisions may reveal more.

John Grunsfeld, assistant administrator of NASA's Science Mission Directorate in Washington, said: "Dark matter is an enigma we have long sought to unravel. With the combined capabilities of these great observatories, both in extended mission, we are ever closer to understanding this cosmic phenomenon."

Carousel credit: NASA/ESA/D. Harvey/R. Massey/HST Frontier Fields


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