Cosmic giants shed new light on dark matter, astronomers say

Mysterious dark matter that pervades the universe behaves as predicted by the 'cold dark matter' theory known as CDM, astronomers have found.
According to a release by the University of Birmingham, UK, astronomers at the University of Birmingham (UK), Academica Sinica in Taiwan, and the Kavli Institute of Physics and Mathematics of the Universe in Japan, have found new evidence that the mysterious dark matter that pervades our universe behaves as predicted by the 'cold dark matter' theory known as CDM.
The astronomers found that the density of dark matter decreases gently from the centre of these cosmic giants out to their diffuse outskirts.

The fall in dark matter density from the centre to the outskirts agrees very closely with the CDM theory.
Almost eighty years after the first evidence for dark matter emerged from astronomy research, few scientists seriously doubt that it exists.
However astronomers cannot see dark matter directly in the night sky, and particle physicists have not yet identified the dark matter particle in their experiments.

"What is dark matter?" is therefore a big unanswered question facing astronomers and particle physicists, especially because there is strong evidence that 85 per cent of the mass in the universe is invisible dark matter.

The team, led by Dr Nobuhiro Okabe (Academia Sinica) and Dr Graham Smith (Birmingham), used the Subaru telescope in Hawaii to investigate the nature of dark matter by measuring its density in fifty galaxy clusters, the most massive objects in the Universe.
"A galaxy cluster is like a huge city that you view from above during the night," explains Smith.
"Each bright city light is a galaxy, and the dark areas between the lights that appears to be empty during the night are actually full of dark matter. You can think of the dark matter in a galaxy cluster as being the infrastructure within which the galaxies live. We wanted to know how the density of dark matter changes as you drive from the centre of a these huge cities out to the suburbs," he said.

The density of dark matter depends on the properties of the individual dark matter particles, just like the density of everyday materials depends on what they are made of.
CDM - the most successful dark matter theory to date - predicts that dark matter particles only interact with each other and with other matter via the force of gravity, they don't emit or absorb light.