Observations of nearby galaxy clusters showed the presence of a light signature with no apparent source. This so-called 3.5 KeV line, named after the energy of its photon, was put forward as a possible signature for a specific type of dark matter, the mysterious substance believed to keep galaxies together.
Dark matter is thought to account for 85 percent of all matter in the universe, however finding it is proving difficult. We know it doesn't interact with light, as we can't see it directly, but it does interact with gravity, as we can see the effect it has on other things. One of the ways we can find out more about it, is to rule out what we know it isn't.
A new study, published in Science, now suggests that this mysterious emission cannot possibly be due to a particular type of dark matter, known as a sterile neutrino. Neutrinos are fundamental particles similar to the electron and its siblings, the muon and the tau, but with no electric charge and with a mass so tiny that it was long thought to be zero. There are three "flavors" of neutrinos (electron, muon, and tau), and they can oscillate between each of them.
A possible fourth type of neutrino, the sterile one, has been proposed to explain several peculiarities of neutrinos and some versions of it fit well with dark matter. As both these sterile neutrinos and dark matter only interact with gravity and no other fundamental forces, looking for them is difficult but they might release a specific light signature when they decay. However, 3.5 KeV line doesn’t appear to be that.
To reach this conclusion, the team used X-ray telescopes of the Milky Way, focusing on the dark region of space surrounding our galaxy. Given that dark matter is expected to be uniformly distributed around galaxies, if it were to emit the 3.5 KeV line, they should have caught it, but they found no evidence of it.
“Our finding does not mean that the dark matter is not a sterile neutrino, but it means that … there is no experimental evidence to date that points towards its existence,” co-author Benjamin Safdi, an assistant professor of physics at the University of Michigan, said in a statement.
The observations were conducted with the XMM-Newton telescope. Astronomers looked at 20 years’ worth of telescope data of these empty spaces around the galaxy. If dark matter was connected to the 3.5 KeV line, they should have seen it close to home.
“Everywhere we look, there should be some flux of dark matter from the Milky Way halo,” co-author Nicholas Rodd, from UC Berkeley, added. “We exploited the fact that we live in a halo of dark matter.”
While this work might be disappointing, it is also a chance to expand the search for dark matter. By focusing on these dark patches and looking at the data of other X-ray observatories on a much wider range of energies, a signature might be detected, giving us some irrefutable proof of the existence of dark matter.