How Old is the Dark Matter?

It is huge, that's for sure, but no one has seen it or knows what it is; it's invisible and light is not emitted or reflected by it. The enigma of the dark matter has been haunting the astronomers since it was first discovered in the 1970s.
It possesses mass and measurable gravitation and an analysis of the galaxies offers a possibility to weigh them; this way, the scientists found that by far the largest matter mass of the galaxy is dark matter.

Galaxies can gather in clusters made of several thousands. The astrophysicist Signe Riemer-Sørensen, PhD student at the Niels Bohr Institute, has investigated two clusters of galaxies colliding.

Around 12 % of the mass belonging to clusters of galaxies is made of enormous clouds of gas and dust; these clouds are the ones that clash, and not the stars or dark matter. The hot clouds spread around x-ray that can be detected and the clouds can be observed as they are expelled out of the two clusters at the collision moment. By colliding, the clouds turn even hotter, emitting more x-ray and an expanding gas front.

Astronomers suggest that the dark matter could be a novel, still undetected particle type, perhaps generated by the decay of the emitted x-ray, like axions. The scientists are interested in places with high concentration of dark matter, but no gas, like spots found in two colliding clusters where the gaseous clouds have been expelled.

Riemer-Sørensen has focused on one of the two colliding clusters, an extremely big one, with many galaxies and up to 85 % dark matter of its overall mass. But there was no x-ray emissions detected.

Dark matter not producing significant x-ray allows the calculation of the speed at which the particles break down, thus the calculation of their lifetime. The measurements showed that if axions represent the dark matter, their lifespan must be over 3.000.000 billion years. The extremely low amount of decayed dark matter suggested 13.7 billion years of age. It seems that dark matter has been there for a long time.