Dark matter is so antisocial, it won鈥檛 even talk to itself. New measurements of collisions between dozens of galaxy clusters show that the mysterious substance is even more ghostly than we imagined.
鈥淥ur results rule out a large proportion of models which predict that dark matter interacts with itself,鈥� says of the Swiss Federal Institute in Lausanne. 鈥淭his is the first time that we鈥檝e been able to analyse this property on such a scale.鈥�
Dark matter is thought to make up around 83 per cent of the matter in the universe, but because it refuses to interact with ordinary matter except through gravity, scientists are still trying to figure out exactly what it is.
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Some researchers have tried to do away with dark matter by modifying the laws of gravity instead, but observations of the Bullet cluster, a collision between two galaxy clusters, suggest that isn鈥檛 possible.
Collision course
Normally, dark matter and regular matter are too well mixed to tell them apart. But when the two clusters collided, their galaxies glided past each other and left a trail of hot, interacting gas behind. The dark matter, seen indirectly by its gravitational effects, remained with the galaxies. This suggests that dark matter particles don鈥檛 bounce off each other like ordinary particles do.
But some studies of other galactic smash-ups have hinted that dark matter might interact with itself via a new, dark-matter-only force. To test this idea, Harvey and his colleagues used images from the Hubble Space Telescope and NASA鈥檚 Chandra X-ray Observatory to look at the positions of gas, galaxies and dark matter in 30 colliding galaxy clusters 鈥� a record number.
鈥淕alaxy clusters have the highest densities of dark matter in the universe, which is why collisions between them make the perfect laboratory in which we can study the self-interacting properties of this mysterious matter,鈥� Harvey says.
They found that when galaxies collide, dark matter carries on its path completely unimpeded and unaffected by any other dark matter around, suggesting that it does not interact with itself after all.
鈥淭his is really exciting work,鈥� says of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淭his is the first time that a very detailed and systematic study of a large number of systems has been undertaken, meaning that we鈥檙e starting to get more precise results. We now really need to look at hundreds or thousands of times more systems.鈥�
Journal reference: Science
