The universe has short-changed us. Less than 5% of it is made up of the stuff we know, the stuff that makes up planets, coffee tables, cars, otters and people. 95% of it is something else entirely and nobody's really sure what that is. About 70% of the universe is postulated to be dark energy and 25% of it is dark matter and it's the latter I'm interested in today.
We know dark matter exists because galaxies and clusters of galaxies simply wouldn't behave the way they do if it didn't. Galaxies would just fly apart is there wasn't something else contributing to their gravity and holding them together. The thing is, gravity is the only force dark matter seems to interact with and that makes it difficult to study. Gravity makes it obvious it's there but it offers up little about what its constituent properties might be.
There is a class of particles called leptons and if those particles were on social media, the electron would have the most followers. It's the only lepton most people have heard of but there are actually six of them. There are two more electron-like particles called the muon and tau and each of these has a corresponding neutrino.
Neutrinos wouldn't be on social media at all because they really don't like to interact with anything. Millions of the things pass through you each day and you just don't notice it. So neutrinos are something that are definitely there but they're hard to spot. That sounds a bit like dark matter and it is logical that scientists might think neutrinos are a good candidate for dark matter.
Some experiments have hinted there might be a fourth neutrino and it has been dubbed the sterile neutrino. Other experiments — observations, really — have detected x-rays coming from distant galaxies and nobody could explain the source of these. Scientists, using no imagination whatsoever, just called this the unidentified x-ray line. Other scientists put two and two together, worked out a way the sterile neutrino might produce the unidentified x-ray line and pitched it as a dark matter candidate.
It all sounds plausible so far but there's a problem. If dark matter is made up of sterile neutrinos and sterile neutrinos produce the unidentified x-ray line, then we should see such a line in our own galaxy. Alas, a recent experiment suggests it's not there. However, some scientists have said this recent experiment is a load of old tosh. Staplers were hurled across rooms in frustration, striking equation-riddled whiteboards. It has caused a bit of a furore.
More observations of this unidentified x-ray line are needed and a satellite launching in 2022 should provide them. Until then, all bets are off and the sterile neutrino may or may not be a candidate for dark matter.