Strange events seen at the very heart of the Milky Way could be smoking gun evidence of a new dark matter suspect. If that is the case, scientists may have been missing the subtle impact of dark matter, the universe’s most mysterious “stuff,” on cosmic chemistry.
This newly proposed dark matter candidate would not only be lighter than existing hypothetical suspects, but it would also be self-annihilating. This means that when two particles of dark matter meet, they destroy each other and create a negatively charged electron and its positively charged equivalent, a positron.
This process and the flood of electrons and positrons would provide the energy needed to strip electrons away from neutral atoms, a process called ionization, in dense gas in the center of the Milky Way. That could explain why there is so much ionized gas in the central region called the Central Molecular Zone (CMZ).
Even if the annihilation of dark matter is rare, it stands to reason it would occur more frequently at the heart of galaxies where it is thought to conglomerate.
“We propose that dark matter lighter than a proton [the particles found in the nuclei of atoms] could be responsible for an unusual effect seen in the center of the Milky Way,” team leader and Postdoctoral Research Fellow at King’s College London Shyam Balaji told Space.com. “Unlike most dark matter candidates, which are often studied through their gravitational effects, this form of dark matter might reveal itself by ionizing gas, essentially stripping electrons from atoms in the CMZ.
“This would happen if dark matter particles annihilate into electron-positron pairs, which then interact with the surrounding gas.”
Dark matter chemistry
Dark matter is thought to account for around 85% of the “stuff” in the cosmos, but despite its ubiquity, scientists can’t “see” it as they do with a great deal of ordinary matter. That’s because dark matter doesn’t interact with light, or if it does, it does so too weakly and too rarely to be observed.
This tells scientists dark matter can’t be composed of the baryonic particles like electrons, protons, and neutrons that compose the atoms that form stars, planets, moons, and everything we see around us on a day-to-day basis.
The only reason scientists theorize that dark matter exists at all is because it does interact with gravity, and this influence impacts light and “ordinary” matter.
This has prompted scientists to look beyond the so-called “standard model of particle physics” to search for particles that could possibly account for dark matter.
These particles vary in mass, given in electronvolts (eV), and in characteristics. It is proposed that some, like this new suspect, could self-annihilate.
The current “leading suspects” for dark matter are axions and axion-like particles, which come in a wide range of masses. However, Balaji and colleagues have mostly ruled out axions and axion-like particles as their dark matter culprits linked to the ionization of gas in the CMZ.
“Most axion models do not predict significant annihilation into electron-positron pairs in the way our proposed dark matter does,” Balaji said. “Our proposed dark matter subject is sub-GeV (one billion eV) in mass and self-annihilates into electrons and positrons.”
“This sets it apart because it directly affects the interstellar medium, creating a signature in the form of extra ionization, something that axions are typically not expected to do.”
Dark matter: It’s own worst enemy
In the densely packed CMZ, created positrons can’t travel far or escape before they interact with nearby hydrogen molecules, stripping away their electrons. That makes this process particularly efficient in this central region.
“The biggest problem this model helps solve is an excess of ionization in the CMZ,” Balaji said. “Cosmic rays, the usual culprits for ionizing gas, don’t seem to be strong enough to explain the high levels of ionization we observe.”
Cosmic rays are charged particles that travel near the speed of light, but according to this team, the ionization signal from the CMZ seems to indicate a slower moving source that is lighter than many other dark matter candidates.
Also, if cosmic rays were ionizing gas in the CMZ, there should be an associated emission of gamma rays, which are very high-energy particles of light. However, this emission is missing from studies of the CMZ.
“If dark matter is responsible for CMZ ionization, it would mean we’re detecting dark matter not by seeing it but by observing its subtle chemical impact on the gas in our galaxy,” Balaji said.
There is, however, an unexplained faint gamma-ray glow from the Galactic Center that might also be linked to positrons and ionization.
“If we find a direct connection between ionization and this gamma-ray emission, it could strengthen the case for dark matter,” Balaji said. “There is some correlation between these two signals, but we still need more data at this stage to say anything stronger.”
Additionally, this annihilation model of dark matter could also explain a signature light emission from the CMZ arising from positively charged positrons and negatively charged electrons smashing together and combining in a state called positronium, which then decays rapidly into X-rays, light with slightly less energy than gamma-rays.
“The numbers fit much better than we expected. Often, dark matter explanations run into issues because they predict signals that should already have been seen by telescopes,” Balaji said. “But in this case, the ionisation rate produced by sub-GeV dark matter fits perfectly within known constraints, without contradicting existing gamma-ray and cosmic microwave background (CMB) observations.”
The researcher added that the preliminary confluence with the X-ray emission is also very intriguing.
“That’s a rare and exciting situation in dark matter research,” Balaji added.
It’s early days for this dark matter suspect
Of course, this new dark matter candidate is at the very beginning of its theoretical lifetime; it doesn’t even have a snappy name like WIMP (Weakly Interacting Massive Particle) or MACHO (MAssive Compact Halo Object) yet!
For comparison, axions have been around since they were first theorized by theoretical physicists Frank Wilczek and Steven Weinberg in 1978.
That means there is a lot of theorizing to be done before this candidate takes its place among axions, WIMPs, MACHOs, primordial black holes, and the rest in the dark matter suspect line-up.
“We need more precise measurements of ionization in the CMZ; if we can map ionization more accurately, we could see whether it follows the expected distribution of dark matter,” Balaji said. “If we rule out other potential sources of ionization, the dark matter hypothesis becomes more compelling.”
Further evidence of a connection between annihilating dark matter and strange emissions from the CMZ could be delivered by NASA’s upcoming COSI (Compton Spectrometer and Imager) gamma-ray space telescope, set to launch in 2027.
COSI should provide better data regarding MeV (1 million eV) scale astrophysical processes, which could help confirm or rule out this dark matter explanation.
Whatever the case, this research has delivered a new way to look at the influence of dark matter.
“Dark matter remains one of the biggest mysteries in physics, and this work shows that we may have been overlooking its subtle chemical effects on the cosmos,” Balaji concluded. ” If this theory holds, it could open up an entirely new way to study dark matter, not just through its gravity, but through the way it shapes the very fabric of our galaxy.”
The team’s research was published on Monday (March 10) in the journal Physical Review Letters.
