Scientists around the world have been speculating on dark matter for decades, but we still don’t know what the mysterious substance is. It doesn’t look like that’s changing any time soon, either. A new study from University of Michigan, Lawrence Berkeley National Laboratory, and the University of California Berkeley evaluated a popular dark matter hypothesis. The team found no evidence that so-called sterile neutrinos are the key to understanding dark matter.
Dark matter is a tricky and sometimes frustrating topic in the physics community. We know dark matter exists and that it makes up about 85 percent of the universe’s total mass. Without that unseen mass, galaxies would fly apart. However, dark matter doesn’t reflect, absorb, or transmit light. The only hints we’ve gotten about dark matter come from the way it interacts with regular matter.
In recent years, scientists have speculated that dark matter may be connected to the neutrino, a neutrally charged particle with very little mass. However, a neutrino with any mass conflicts with the standard model of particle physics. Researchers have put forward the idea of a “sterile neutrino” that could both explain neutrino mass and be the source of all that unseen dark matter.
The new analysis sought to find supporting evidence for the sterile neutrino in 20 years of historical data from the XMM-Newton space X-ray telescope. Based on our limited understanding of dark matter, this material collects in halos around galaxies. Rather than looking at far away objects, the team looked closer to home, attempting to confirm a 2014 observation that gave credibility to the sterile neutrino theory.
A successful detection would have included data points along the dotted red line, but the team found none.
That 2014 study described a 3.5 keV signal from nearby galaxies that could have come from decaying sterile neutrino dark matter. For the new study, the team exploited the fact that we live inside the halo of dark matter that surrounds the Milky Way. They analyzed data from the “darkest” parts of the Milky Way in search of that same 3.5 keV signal. They didn’t find it.
This study doesn’t necessarily mean sterile neutrinos don’t exist or that they aren’t a key source of dark matter. However, it does suggest that the scant experimental evidence for this theory doesn’t stand up to scrutiny. It’s back to the drawing board in our search for dark matter.
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