Scientists Search for Enigmatic Dark Matter in the Milky Way

Dark matter makes up 85% of the universe, but it is not known what exactly it is. Now, a recent study performed by the University of Michigan, the University of California, Berkeley, and Lawrence Berkeley National Laboratory (Berkeley Lab) has ruled out the fact that dark matter accounts for the enigmatic electromagnetic signals that were earlier visualized from neighboring galaxies.

Before this study was conducted, there was tremendous hope that these electromagnetic signals would offer solid proof to physicists to help detect dark matter. It is not possible to directly observe dark matter because it does not emit, reflect, or absorb light; however, scientists are aware of its existence because of the impact it has on other matters.

For instance, dark matter is required to elucidate the gravitational forces that are responsible for holding the galaxies together.

It has been suggested by physicists that dark matter is a closely associated cousin of the neutrino, known as the sterile neutrino. Neutrinos are subatomic particles that do not carry any charge and which seldom communicate with matter. These particles are discharged during nuclear reactions that occur within the Sun. They have a negligible amount of mass, but this mass has not been elucidated by the Standard Model of Particle Physics.

Physicists have proposed that a hypothetical particle called sterile neutrino could be responsible for this mass and could also be dark matter.

According to Ben Safdi, the co-author of the study and an assistant professor of physics at the University of Michigan, scientists should be able to spot the sterile neutrino since it is not stable. The sterile neutrino decays into normal electromagnetic radiation and neutrinos. Hence, to identify dark matter, physicists subsequently scan galaxies to search for this electromagnetic radiation in the form of X-ray emission.

Back in 2014, a pivotal work identified the surplus emission of X-rays from neighboring galaxies as well as clusters of galaxies. The X-ray emission seemed to be consistent with that of the disintegrating sterile neutrino dark matter, explained Safdi.

Over a period of two decades, a meta-analysis was performed on the raw data of objects present in the Milky Way galaxy captured by XMM-Newton space X-ray telescope. Now, according to this meta-analysis, there is no proof that the sterile neutrino is composed of dark matter.

The team of researchers includes Christopher Dessert, doctoral student from the University of Michigan, and Nicholas Rodd, a physicist with the Berkley Lab theory group and the Berkley Center for Theoretical Physics. The team’s results were published in the Science journal.

This 2014 paper and follow-up works confirmed the signal generated a significant amount of interest in the astrophysics and particle physics communities because of the possibility of knowing, for the first time, precisely what dark matter is at a microscopic level. Our finding does not mean that the dark matter is not a sterile neutrino, but it means that—contrary to what was claimed in 2014—there is no experimental evidence to-date that points towards its existence.

Ben Safdi, Study Co-Author and Assistant Professor, Department of Physics, University of Michigan

Space-based X-ray telescopes, like the XMM-Newton telescope, point at environments rich in dark matter, to look for this trace electromagnetic radiation in the form of X-ray signals. The discovery made in 2014 dubbed the X-ray emission as the “3.5 keV line”—keV is short for kilo-electronvolts—because of where the signal emerged on X-ray detectors.

Using two decades of archival data captured by the XMM-Newton space X-ray telescope, the researchers looked for this line in the Milky Way galaxy. Physicists are aware that dark matter accumulates around galaxies; hence, when earlier analyses looked at neighboring galaxies and clusters of galaxies, all of those images would have recorded some column of the dark matter halo in the Milky Way.

The researchers utilized those images to observe the “darkest” portion of the Milky Way galaxy. This considerably enhanced the sensitivity of earlier analyses that searched for sterile neutrino dark matter, added Safdi.

“Everywhere we look, there should be some flux of dark matter from the Milky Way halo,” stated the Berkeley Lab’s Rodd, because of the location of the solar system in the galaxy. “We exploited the fact that we live in a halo of dark matter” in the research.

According to Christopher Dessert, the co-author of the study and a physics researcher and PhD student at the University of Michigan, galaxy clusters where the 3.5 keV line have been visualized, also have massive background signals, which act as noise in observations and can render it hard to pinpoint certain signals that may be related to dark matter.

The reason why we’re looking through the galactic dark matter halo of our Milky Way galaxy is that the background is much lower.

Christopher Dessert, Doctoral Student, University of Michigan

For instance, XMM-Newton has captured images of separated objects, such as individual stars in the Milky Way galaxy. The scientists captured these images and concealed the objects of original interest, leaving behind dark and pristine environments in which to look for the glow of the decay of dark matter.

Integrating two decades of such visualizations made it possible to inspect sterile neutrino dark matter to unparalleled levels. If sterile neutrinos were actually dark matter, and if their decay caused the emission of the 3.5 keV line, then Safdi and his fellow team should have visualized that line in their study. However, they did not find any such proof for sterile neutrino dark matter.

While this work does, unfortunately, throw cold water on what looked like what might have been the first evidence for the microscopic nature of dark matter, it does open up a whole new approach to looking for dark matter which could lead to a discovery in the near future.

Ben Safdi, Study Co-Author and Assistant Professor, Department of Physics, University of Michigan

Scientists in this work were supported by the U.S. Department of Energy’s Early Career Research Program, Leinweber Center for Theoretical Physics at the University of Michigan and Miller Institute for Basic Research in Science at UC Berkeley.

The study was financially supported by Advanced Research Computing at the University of Michigan.