An unusual gamma ray signal could be the most direct evidence of dark matter to date. However, the signal could also be the result of a telescope anomaly or be produced by something even stranger than dark matter.
Dark matter forms the fundamental structure of much of the universe, determining how galaxies cluster together, for example. It also vastly exceeds all visible matter in mass, but for decades researchers have puzzled over exactly what it is.
Part of the problem is that they cannot observe dark matter directly. Instead, they can only record its effects on the surroundings. A more direct line of research would be to detect particles produced when dark matter collides and annihilates itself, which some theories suggest should occur. Now, Yun-Feng Liang of Guangxi University in China and his colleagues have identified a gamma ray signal that could result from this process.
The researchers found the signal after analysing 15.5 years of data from the Fermi Gamma-ray Space Telescope (FGST), specifically examining the galaxy clusters Virgo, Fornax and Ophiuchus. The researchers chose these galaxy clusters because they are known to contain large dark matter haloes, according to Liang. The gamma ray signal the team found there had the shape of a peak or line, analogous to a flash of light of a single colour, which some theories predict results from dark matter annihilating itself.
"Discovering a clear gamma ray line would be the definitive 'smoking gun' evidence proving the existence of dark matter particles and revealing their properties in particle physics," says Yi-Zhong Fan, a team member from the Chinese Academy of Sciences. Based on their statistical analysis of the data, the researchers estimated that there is less than a 1 in 10,000 chance that this signal is random cosmic noise or a purely coincidental pattern.
However, several questions remain before the team can declare a definitive discovery. Detecting dark matter signatures in gamma rays has historically been difficult because signals tend to be weak and can be confused with telescope errors. A promising gamma ray signal from 2012, for example, turned out to be exactly that. Zhao-Qiang Shen, also from the Chinese Academy of Sciences and who worked on the new study, says that team members performed a large number of tests to confirm the authenticity of their signal, but some possibility of instrument error remains.
Moreover, although the new signal is clear when the three galaxy clusters are observed together, it appears to fade closer to the centre of our galaxy, where dark matter is dense and should also be annihilating and creating gamma rays.
"The fact that the signal lights up in distant clusters but remains silent in our own cosmic backyard is profoundly peculiar," says Liang. "If this truly comes from dark matter, it suggests that particles must interact in a much more sophisticated way than conventional theories predict." Alternatively, the signal could come from some even rarer or more novel phenomenon, such as ultra-fast particle winds from exotic magnetised neutron stars.
"I find the result intriguing, but would be quite cautious in interpreting it as evidence of dark matter," says Juri Smirnov of the University of Liverpool in the United Kingdom. He points out that observing only one galaxy cluster rather than the three together increases the odds that the signal is the product of noise or a random pattern. He is also concerned about how unconventional a dark matter process would have to be to produce this gamma ray peak far from us but not near the centre of our galaxy. "This puts the standard dark matter interpretation under considerable strain," he says.
The team hopes that future telescope missions will add clarity to the situation, for instance the Very Large Area Gamma-ray Space Telescope, which was proposed several years ago and could collect more gamma rays with better resolution. By 2040, the FGST will also have doubled its dataset, which could further elucidate the nature of the strange gamma ray peak, according to Fan.
"If future telescopes confirm that this signal is genuine, it would represent a historic breakthrough. It would immediately reveal the mass of the elusive dark matter particle and give physicists a concrete target for building the next generation of fundamental particle theories," says Shen. "At the same time, it would force us to completely rewrite our standard textbook models of dark matter".
