In spite of substantial scientific investigation and convincing indirect evidence, dark matter still eludes direct detection and its existence essentially remains a tantalizing, but unproven, hypothesis. Notwithstanding this, nearly 85 percent of the predicted mass of the universe remains unaccounted for, and dark matter theory is still the prime contender to explain where it may be. Researchers at the University of Southampton have theorized the existence of a new "lighter" dark matter particle in an effort to help unravel the mystery.

Physicists believe that dark matter exists because of a number of tell-tale pieces of evidence gathered over the years that tend to fit with the theory. These include observed gravitational effects on stars and galaxies – including the phenomenon of gravitational lensing where light rays are bent around large unseen masses in space – and apparent evidence of anistropic patterns in the Cosmic Microwave Background, which alludes to its existence.

Many scientists believe that dark matter particles have a very large mass, similar to that of heavy atoms. The idea of lighter dark matter particles has not been considered likely on the basis of current observed astrophysical properties of space particles.

However, when researchers from a number of diverse physics disciplines at the University of Southampton were tasked with investigating the light dark matter theory a previously unknown opening in the science appeared. It was found that such an idea could exist and (along with some quite general arguments from particle physics) the researchers arrived at some surprising results.

"This work brings together some very different areas of physics: theoretical particle physics, observational x-ray astronomy, and experimental quantum optics," said Doctor James Bateman, from Physics and Astronomy at the University of Southampton. "Our candidate particle sounds crazy, but currently there seem to be no experiments or observations which could rule it out. Dark Matter is one of the most important unsolved problems in modern physics, and we hope that our suggestion will inspire others to develop detailed particle theory and even experimental tests."

The theoretical particle that would fit the new hypothesis suggested by the interdisciplinary teams would have a mass of around 100eV/c2 (1.78266184 × 10-34 kilograms), which is approximately 0.02 per cent of the mass of an electron. This newly-supposed (and yet un-named) fundamental particle apparently would not interact with light but would strongly interact with normal matter. And, unlike other proposed dark matter candidates, it may not even be able to penetrate Earth’s atmosphere due to its infinitesimally small mass.

As a result, the team suggests that Earth-based detection of the particle would not be likely, so they intend to piggyback on searches being proposed for an experiment in space involving a Macroscopic Quantum Resonator (MAQRO) that aims to explore untested parameters of quantum physics by observing the decoherence of superpositions of macroscopic objects.

Put simply, the MAQRO experiment intends to use the incredibly low temperatures and ultra-high vacuum of space combined with an optical trap to explore the quantum-mechanical concept of superposition for massive particles. As the new light matter particle has been defined by the team as a nanoparticle, it is envisaged that the MAQRO experiment will help expand the understanding of the quantum behavior of similar objects in space.

The researchers believe that the new nanoparticle would be found suspended in space and be part of the direct flow of dark matter, thereby being pushed along by the mass. Subsequent detection and monitoring of this particle’s position is then expected to reveal evidence about the nature of this dark matter particle, provided that it actually exists.

"At the moment, experiments on Dark Matter do not point into a clear direction and, given that also the Large Hadron Collider at CERN has not found any signs of new physics yet, it may be time that we shift our paradigm towards alternative candidates for Dark Matter," said Doctor Alexander Merle from the Max Planck Institute, and co-collaborator on the research. "More and more particle physicists seem to think this way, and our proposal seems to be a serious competitor on the market."

The research is published in the Nature journal Scientific Reports