Is dark matter made of mysterious 'ghost particles?' Galaxy clusters could hold the answer

If dark matter particles decay, then scientists could hunt for signs of this process, including X-ray or gamma-ray radiation or even emitted "ghost particle" neutrinos, in vast clusters of galaxies.

Not only could this finally reveal what particles comprise mysterious dark matter, but it could also help astronomers understand the universe's structure like never before. And new research suggests that NASA's X-ray Imaging and Spectroscopy Mission (XRISM) could play an important role in this hunt.

Dark matter poses a significant challenge for scientists because, despite comprising around 85% of the matter in the cosmos, it remains effectively invisible. This is because it doesn't interact with electromagnetic radiation, or light — or, if it does, the interaction is too weak to be detected. This has led scientists to suggest a whole host of hypothetical particles to account for dark matter, which go beyond the standard model of particle physics and the electrons, protons and neutrons that make up the atoms that compose all everyday matter, like stars, planets, moons and our bodies.

One particular dark matter model suggests that whatever particles make up this mysterious stuff, they undergo a process called decay. This involves large particles breaking down over vast timescales to lighter particles, releasing energy in the form of photons, the particles of light. One possible signature of this process that astronomers could hunt for are X-ray photons released when decay occurs. In fact, scientists may have already spotted this cosmic fingerprint in the form of an unidentified X-ray emission in the light spectra from galaxy clusters.

"Eighty-five percent of mass in galaxy clusters comes from dark matter, and we can model the dark matter radial distribution well," study team member Ming Sun, of the University of Alabama in Huntsville (UAH), said in a statement. "Thus, galaxy clusters are great targets for such a search as they are dark matter-rich and we know the dark matter mass in clusters well."

In the past, researchers have relied on light-sensitive semiconductor chips called Charge-Coupled Devices (CCDs) to track the paths of possible decay particles to better understand what is causing this X-ray emission. However, Sun and colleagues took a different approach, instead turning to data from XRISM.

"Nearly all the past studies used the CCD data, which lack the required energy resolution to resolve the unidentified line," Sun said. "Now XRISM provides high-energy-resolution spectra that can resolve the line. As the line signals are very weak, we combined nearly three months of the XRISM data for such a search. There are many X-ray lines detected. They originate from known atoms, such as iron, silicon, sulfur, and nickel. X-ray emission lines that appear that are not at the known position of atomic lines are then the candidates for dark matter decay lines, which is the focus of this work."

The team theorizes that the leading suspects for this unknown emission are "sterile neutrinos." Neutrinos are virtually massless particles that stream through the cosmos at nearly the speed of light. The second-most abundant particle in the universe after photons, neutrinos are so "ghost-like" that 100 trillion pass through your body every single second, and you never notice a thing. Sterile neutrinos are one of the hypothetical particles that have been proposed to account for dark matter.

"A sterile neutrino is a hypothetical type of neutrino that only interacts with other particles via gravity, unlike the three known 'active' neutrinos that also interact via the weak force," Sun said. "The existence of the sterile neutrino is well-motivated theoretically and can explain the very small but non-zero mass of regular neutrinos. Sterile neutrinos can decay into two photons with the same energy. Models can predict the decay rate of sterile neutrinos, which is then constrained from the data."

Sterile neutrinos have a long way to go before they replace Weakly Interacting Massive Particles (WIMPs) as the leading suspects for dark matter, but Sun and colleagues are committed to exploring other possible candidates, including sterile neutrinos, even if that process includes ruling them out.

"WIMPs are still the leading candidate for dark matter, but billions of dollars of experiments have been done, only getting stronger and stronger upper limits, so alternative scenarios have to be considered. This study provides the strongest limits from high-energy-resolution data on the sterile neutrino at the 5 to 30 kiloelectronvolts (keV) band, subsequently limiting the models for dark matter," the UAH researcher concluded. "With more XRISM data in the next five to 10 years or so, we will be able to either detect the line or improve the limit substantially."

The team's research was published in November in The Astrophysical Journal Letters.