A map of the gamma ray background with the rays coming from our own galaxy turned masked with grey. Credit: Mattia Fornasa/UvA/Grappa

There’s No Trace of Dark Matter Inside The Gamma Ray Background, Researchers Find

The search continues.

20 DEC 2016

Scientists have found no trace of dark matter inside the most precise analysis of the gamma-ray background to date.

The gamma-ray background is the name given to the strange, unexplained trace signature of gamma rays found across the Universe. It was thought that these gamma rays could be associated with the equally mysterious dark matter, but a new study has now shown that something else is going on. 


Gamma rays are photons that are invisible to the human eye, but have the highest amount of energy of all photon types in the known Universe.

They're so energetic that they're ionising - capable of changing our cells as they pass through - and since the 1960s, scientists have detected a mysterious, diffuse background of gamma rays throughout the Universe.

So far, researchers have managed to find roughly 3,000 extragalactic sources of this vast gamma-ray background, but these sources do not account for the amount of gamma rays we know exist.

In fact, upwards of 75 percent of the gamma rays in the Universe have unknown sources.

Some known sources of gamma rays include blazars, which are ultra-condensed regions surrounding supermassive black holes that blast out enormous amounts of electromagnetic radiation. They've also been seen coming from pulsars and supernovae.

Other than that, scientists are stumped, and those sources can't explain why there's so much gamma-ray background.


But the gamma rays have to be coming from somewhere because, as the team puts it, "If you had gamma-ray vision, and looked at the sky, there would be no place that would be dark."

If this problem sounds familiar, it’s because the gamma ray knowledge gap is a lot like the mystery of dark matter.

The issue there is that the amount of observable matter in the Universe doesn’t add up to the amount of gravity we know exists, particularly within galaxies.

So to fix that, researchers came up with dark matter - a hypothetical type of matter that's invisible because it doesn't interact with electromagnetic radiation, and can explain why there's so much gravity out there. The only problem is we so far haven't been able to detect dark matter.

Because dark matter could explain the amount of gravity in the Universe, researchers have also considered that it might explain other weird things, too, such as the amount of gamma rays we know exist.

To see whether this was the case, the team - led by Mattia Fornasa from the University of Amsterdam in the Netheralands - used high-powered telescopes such as NASA’s Fermi Large Area Telescope to scan the gamma-ray background in search of fluctuations that might provide clues about where it came from.

The team analysed 81 months’ worth of data collected by the orbiting Fermi Large Area Telescope.

Looking at these fluctuations, they found that the gamma-ray background is generally made up of two types of gamma rays: those that came from high-energy sources, and those that came from low-energy sources.

So where did they come from? 

Based on their analyses, the team found evidence that some of these high-energy gamma rays could have formed from blazars that we're yet to discover - a hypothesis that is currently undergoing further study by a separate group of scientists.

The lower-energy rays, though, are still a mystery. The team does note that none of the previously known sources of gamma rays - such as blazars, pulsars, and supernovae - could produce them.

And dark matter couldn't either - the researchers found no evidence to support dark matter decay or annihilation playing a role in the creation of the gamma-ray background after analysing their findings with complex computer simulations.

"Our measurement complements other search campaigns that used gamma rays to look for dark matter and it confirms that there is little room left for dark matter induced gamma-ray emission in the isotropic gamma-ray background," said Fornasa.

The study has been accepted for publication in an upcoming edition of Physical Review D. Until then, you can read the preprint version at arXiv.org.

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