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Dark matter is ghostly and non-interactive

Colliding galaxies A new study of colliding galaxy clusters has found that dark matter doesn't even interact with itself.

The findings reported in the journal Science, mean some existing dark matter models - which give the mysterious substance properties similar to normal matter - will need to be revised.

"We have concluded that dark matter is most probably not interacting, so it exists in its ghostly state without interacting," says the study's lead author Dr David Harvey of the École Polytechnique Fédérale de Lausanne in Switzerland.

"This is surprising because we see in our world that all the particles interact with each other quite highly, whereas dark matter does not seem to do that."

Astronomers first noticed dark matter when they realised that there wasn't sufficient gravitational attraction to keep stars orbiting as fast as they do around the centres of galaxies.

Another apparently invisible substance, which scientists now call dark matter, must be providing the additional gravity.

Scientists estimate that dark matter makes up 85 per cent of all the matter in the universe.

All the normal matter - which makes up all the stars, planets, dust and gas clouds (which scientists call baryonic matter) - makes up just 15 per cent of all the matter in the universe.

Gravitational lensing

The new research by Harvey and colleagues, examined 72 galaxy cluster collisions to see how dark matter interacts.

Galaxy clusters are huge, gravitationally-bound collections of galaxies - interspersed with immense clouds of gas - which form some of the largest structures in the universe.

The authors compiled optical and X-ray images of galaxy cluster collisions using data from the Earth orbiting Chandra X-ray observatory and Hubble Space Telescope.

The X-rays are emitted by gas allowing scientists to pinpoint where the gas clouds are located, while the optical data shows the location of galaxies.

"Hubble allows us to see the galaxies in the galaxy cluster and also look at the galaxies behind galaxy clusters," says Harvey.

"By looking at background galaxies behind the cluster, you can see how light from those galaxies is bent by the mass of the foreground cluster.

The way light is bent provides clues about where the cluster's dark matter is located, and how it interacts during collisions.

The authors found galaxies pass through each other unimpeded during collisions, with their movement controlled by gravity.

They also found that the gas of each galaxy cluster interacts with the gas of the colliding cluster as they merge, slowing down and separating from its original galaxy cluster.

The question is; what is the dark matter doing during these collisions?

"We found the dark matter doesn't slow down, so as these huge dense lumps of dark matter come together, they go through each other without any interaction, they just follow the galaxies, or more accurately the galaxies are sticking to the dark matter," says Harvey.

"This is telling us that dark matter will most likely not interact the way protons [of normal matter] do, so it's ruling out models of dark matter that try to mirror the universe we have.

"Various dark matter models predict that dark matter will self interact to a certain degree, but what we've shown is that it doesn't."

Time for a rethink

According to Harvey, theorists will now need to tweak their dark matter models of 'mirror universes' and 'dark photons' in order to match his teams observations.

"At the moment there are inconsistencies ... what we are doing is getting us closer to understanding what dark matter is," says Harvey.

The authors work complements the dark matter research about to be undertaken by the Large Hadron Collider at CERN.

"What CERN and the other ground based detector experiments are looking to do is see how dark matter relates to the standard model of particle physics," says Harvey.

"What I'm looking at is how dark matter interacts with in its own dark sector, its own dark universe side, which you can't do from the ground."