Whilst probing the elemental composition of various galaxy clusters, scientists at the Harvard-Smithsonian Center For Astrophysics (CfA) discovered an unusual emission line that they’re struggling to identify. While confirmation from other observatories is required, they have proposed an exciting idea to explain this mysterious signal: it could be from the decay of a dark matter particle.
Despite the fact that it makes up around 27% of the universe, scientists know little about dark matter. In contrast with normal matter, which makes up around 5% of the universe, dark matter doesn’t interact with the electromagnetic force. This means that it doesn’t absorb, reflect or emit light and hence is invisible. Scientists are only aware of the existence of dark matter because it exerts gravitational effects on visible matter.
So what is dark matter? Good question. Scientists have a reasonable idea of what dark matter is not, rather than what it is. They don’t think that it’s made up of baryons such as protons and neutrons which make up the bulk of normal matter. They’re also pretty sure it’s not antimatter or large black holes. The most widely accepted idea is that it’s made up of exotic particles such as Weakly Interacting Massive Particles.
While it’s notoriously tricky to detect, clusters of galaxies bound together by gravity are thought to be the prime location for dark matter spotting. The space between galaxies is packed with scorching hot intergalactic gas that can exceed temperatures of 10 million kelvin. This gas contains various heavy elements such as oxygen, magnesium, argon and iron that can be detected by looking at their X-ray emission lines. Scientists are interested in the abundances of these elements because they can provide clues about the rate of supernova explosions in different galaxies since these events throw the elements out into the gas. It’s also believed that this X-ray emitting intracluster gas may provide us with dark matter signatures, but so far no one has been so lucky. Now, however, CfA astronomers may be tantalizingly close to finally lifting the veil on this elusive matter.
Whilst investigating the averaged X-ray spectrum of 73 galaxy clusters, scientists discovered a weak line which didn’t appear to match up with any of the known elements. Furthermore, the line was located in a spectral interval that was not expected to possess any features.
The team has suggested in their recently published paper that this unidentified signal could be the result of the decay of a long-sought dark matter candidate particle called a sterile neutrino. More work needs to be done at this stage to separate the signature from background noise, but if the result can be confirmed with observatories that can provide better resolution, such as the upcoming Japanese Astro-H X-ray observatory, it would be a breakthrough. However, until that data is collected the scientists cannot rule out the possibility that the emission line is merely the result of some kind of error.