• Researchers look for dark matter close to home (U Michigan)
• How Do We See Dark Matter? (NASA)
• New Technique Looks for Dark Matter Traces in Dark Places (Berkeley Lab)
• Sterile Neutrino (ArXiv pdf)

It is spring and as we all shelter in place, the excess of daylight is easy to miss. It seems only fitting that in this dark time we should take a moment to talk about dark matter.

When we look out at the motions of large and larger objects, we see behaviors that can only be explained if there is more material out there than we can see without telescopes. Discovered by both Vera Rubin and Fritz Zwicky, who were looking at galaxies and galaxy clusters, today we know that at least 25% of the mass-energy of the universe is hidden in this mysterious stuff. Initially, it was hoped that if we just look harder and in new wavelengths, we’d uncover this stuff in the form of gas, dust, black holes, and rogue planets. When I was in graduate school, the emptiness of our universe and the scale of this problem was proud home to me when professor Don Winget pointed out that you could account for all the dark matter by placing one acme brick in each solar system sized volume of space. Our universe is seriously empty, and we’re looking for stuff that adds up over the size of the universe, but in general just isn’t all that much stuff.

In the 1990s and early 2000s, teams searched for black holes and other dark objects that might be hiding out in plain sight. Specifically, they looked toward our galactic bulge, and toward the Magellanic Clouds and looked for the gravity of nearby objects to gravitational lens the distant stars and make them temporarily seem brighter. While these projects did see gravitationally lensed things, including at least 1 planet! 

What they found wasn’t nearly enough to account for all dark matter effects we see. At the same time that observers were ruling out normal things being dark matter, other astronomers were thinking through how clouds of dark matter particles could gravitationally reshape images of distant galaxies, allowing us to look for dark matter that was nothing more than a distribution of particles – and we found this. The Bullet Cluster image was perhaps the most famous example of how dark matter has been mapped by looking at how it reshapes our view of what lurks behind it.

The only thing is, these particles don’t seem to interact through the electro-magnetic force in normal ways – they don’t produce or interact with light except through gravitational tugs on photons. We don’t have reason to think that it interacts with magnetic fields, and in general, most of our normal ways of seeing things are defeated by dark matter.

And dark matter’s way of hiding only makes us want to find it all that much more, and astronomers and physicists around the world are struggling to find new ways to detect patricals.

In a new study from a collaboration by folks at the University of Michigan, Lawrence Berkeley Labs, and UC Berkeley sought to see if maybe, mysterious flickers of light from nearby galaxies might be caused by dark matter. These flickers are believed to be from a theorized particle called a sterile neutrino, and they appear as XRay flickers in massive galaxies. It was thought that decay of sterile neutrinos might be producing this line.

This begs the question, what is a sterile neutrino? We have no experimental evidence that requires these particles to exist, but most observed particles in the standard model come in different varieties that complimentary spin. All observed neutrinos to date have a left-handed spin, sterile neutrinos are theorized as the right handed spinning version of this hard to study little particle – a particle that acts an awful lot like dark matter in terms of not interacting. It was hoped that the sterile neutrino would prove real and prove to be at least part of the solution for dark matter.

Analysis of data from the XMM-Newton looking at the flickers in large galaxies to see if they could be proven to be caused by sterile neutrino decay was consistent with big galaxies having lots of decays to go with their lots of dark matter. This should mean that smaller galaxies, like our own Milky Way, should have the same flickers in proportion to the size of our galaxy. The thing is… observations don’t show that. By going through all the archives of XMM data they could, this team searched near and far in our Milky Way and there were no flickers to be seen. This indicates that whatever causes these flickers is unique to the environment of large galaxies – and dark matter isn’t unique to any location, so… the flickers and dark matter can’t be linked in a way we can understand.

We may still be able to blame sterile particles for dark matter, but the flickers aren’t sterile neutrinos decaying.