A sketch (not to scale) showing axions (blue) streaming out from the Sun, converting in the Earth's magnetic field (red) into X-rays (orange), which are then detected by the XMM-Newton observatory. University of Leicester

Scientists at the University of Leicester have detected what appears to be a signature of dark matter particles in what could be a direct observation of the 'invisible' mysterious stuff that comprises 85% of matter in the universe.

A curious signal in the X-ray sky has shown characteristics of 'axions', the predicted dark matter particle candidates.

The team discovered a seasonal signal in the X-ray background which usually looks unchanged whenever studied.

The X-ray background is the sky, after the bright X-ray sources are removed.

The findings are the result of an extensive study of almost the entire archive of data from the European Space Agency's X-ray observatory, XMM-Newton, 15 years in orbit now.

Professor George Fraser, late Director of the University of Leicester Space Research Centre.

Late Professor Fraser explains in the paper, "It appears plausible that axions – dark matter particle candidates - are indeed produced in the core of the Sun and do indeed convert to X-rays in the magnetic field of the Earth."

The X-ray signals due to axions are expected to be greatest when looking through the sunward side of the magnetic field because that is where the field is strongest.

Dark Matter cannot be seen directly with telescopes, but is instead inferred from its gravitational effects on ordinary matter and on light.

Without the presence of this invisible stuff, most of the matter in the universe would have been flung apart. The gravity of dark matter is what holds the universe together.

A recent discovery of an x-ray radiation which could be emitted by another dark matter candidate, a sterile neutrino, was reported by Harvard scientists. Data from the ISS has also thrown some light on the nature of dark matter with possibilities of positrons being produced in collisions between dark matter.