Astronomers have long been intrigued by two puzzling phenomena at the center of our galaxy. First, the gas in the so-called Central Molecular Zone (CMZ)—a dense and turbulent region near the Milky Way’s core—appears to be ionized at a surprisingly high rate. That means atoms are losing electrons and becoming electrically charged.
Second, scientists have detected a mysterious gamma-ray signal with an energy of 511 kiloelectronvolts (keV), which matches the rest energy of an electron.
This specific gamma-ray emission occurs when an electron meets its antimatter counterpart, the positron, and they annihilate in a brief burst of light.
Despite years of observation, the origins of both of these signals have remained elusive.
Researchers propose in a new study published in Physical Review Letters that dark matter — one of the universe’s most elusive components — may link both mysteries. Specifically, they suggest that a new, lighter form of dark matter than typically considered could be responsible.
A hidden process at the galactic center
The CMZ stretches almost 700 light-years and contains some of the densest molecular gas in the galaxy. Researchers have observed that this region ionizes unusually fast, splitting hydrogen atoms into charged particles at unexpectedly high rates.
Starlight and cosmic rays contribute to this ionization, but they don’t seem to explain the observed intensity.
The 511 keV gamma-ray signal, first detected in the 1970s, also lacks a definitive source. Researchers have suggested candidates like supernovae, massive stars, black holes, and neutron stars, but none fully explain the emission’s intensity and distribution.
This led researchers to ask a key question: Could both phenomena stem from the same hidden cause?
Enter dark matter
Dark matter makes up around 85% of the matter in the universe, but it doesn’t interact with light. While its gravitational effects are well-documented, its true nature remains unknown. One underexplored possibility is that dark matter particles could be extremely light—just a few million electronvolts in mass, far lighter than a proton. These particles are known as sub-GeV dark matter.
According to the new study, these light dark matter particles could annihilate with their own antiparticles at the galactic center, releasing electrons and positrons. In the dense CMZ gas, those low-energy particles would rapidly lose energy and efficiently ionize surrounding hydrogen by knocking off electrons—closely matching observed ionization patterns.
Detailed simulations showed that this annihilation process can naturally account for the ionization levels seen in the CMZ, and crucially, the required properties for this dark matter do not conflict with constraints from the early universe. That makes it a strong candidate.
The positron puzzle
If dark matter is indeed generating positrons in the CMZ, those particles will eventually slow down and annihilate with electrons, producing 511 keV gamma rays. This would directly link the two mysterious signals.
The study found that while dark matter alone explains the ionization, it could also contribute to some of the 511 keV emission. This striking possibility suggests that both signals might share the same source—light dark matter.
However, the brightness of the gamma-ray signal depends on several still-uncertain factors, such as how and where positrons annihilate and how efficiently they form bound states with electrons.
A new way to study the invisible
Whether or not the two signals share a common source, the CMZ’s ionization rate is emerging as a powerful tool to study dark matter—especially lighter particles that are difficult to detect with traditional lab-based experiments.
The study showed that the predicted ionization profile from dark matter is remarkably flat across the CMZ, which aligns with actual observations.
Point sources like the central black hole or cosmic ray sources (such as supernovae) can’t easily produce this uniform profile—but a smoothly distributed dark matter halo can.
These findings suggest that the center of the Milky Way may hold important clues about the nature of dark matter.
Future telescopes with improved resolution may be able to clarify the spatial connection between the 511 keV line and the ionization rate in the CMZ. Meanwhile, ongoing observations could help confirm—or challenge—the dark matter hypothesis.
Either way, these curious signals from the heart of the galaxy remind us that the universe still holds many surprises. Sometimes, looking inward—toward the vibrant and dynamic core of our own galaxy—reveals the most unexpected hints of what lies beyond.
Read the original article on: Science Alert
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