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A New Hypothesis About Dark Matter May Offer Answers to Some of the Greatest Enigmas of Our Galaxy. Researchers Suggest That an Alternative Form of This Invisible Substance Could Explain the Unusual Distribution of Stars and Gravitational Patterns in the Milky Way
Scientists have been observing two intriguing phenomena at the heart of the Milky Way for decades. One of them is the surprisingly high ionization rate of gas in the so-called central molecular zone (CMZ), a dense and chaotic region near the galactic core.
The other is a persistent glow of gamma rays with an energy of 511 keV (kilo-electronvolts), identified by telescopes since the 1970s.
Now, a new study published in the journal Physical Review Letters proposes that both effects may have a common cause: a light and previously underappreciated form of dark matter.
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Excessively Ionized Gas
The CMZ extends for approximately 700 light-years, composed of some of the most dense molecular gases in the galaxy.
One piece of data caught the attention of astronomers: the gas present there is strongly ionized. This means that hydrogen molecules are being broken down into charged particles (electrons and nuclei) at a rate much higher than expected.
This type of ionization could be caused by cosmic rays or radiation from stars, but these known sources do not explain the levels recorded in the CMZ. The observed intensity is above what these traditional mechanisms would be capable of producing.
Gamma Rays Without Clear Source
The second mystery is the glow of 511 keV. This specific radiation is emitted when an electron meets its antiparticle, the positron. Upon colliding, the two annihilate, releasing this characteristic energy in the form of light.
The origin of these positrons, however, remains unclear. Explanations such as supernovas, black holes, and neutron stars have been suggested. None, to date, have fully justified the pattern or intensity of the observed glow.
The Question That Led to the Hypothesis
With this data in hand, researchers decided to investigate a new possibility: what if both phenomena, the ionization and the gamma rays, are linked to the same hidden process?
The proposal involves an unlikely candidate, but one that is starting to gain traction: a light form of dark matter.
This type of particle, lighter than a proton, has been little studied until now. But it may represent a significant fraction of the invisible matter that makes up 85% of the universe.
These particles, called sub-GeV dark matter (less than one giga-electron-volt), could annihilate with their own antiparticles, generating electrons and positrons in the process.
Energy Released in the Right Place
The team of scientists simulated how these particles would behave in the CMZ. Being an extremely dense region, the energy released by these annihilations would be quickly absorbed by the surrounding gas. This local effect would cause hydrogen molecules to be ionized very efficiently — exactly what is observed in the CMZ.
The simulations showed that this hypothesis works well. The predicted ionization profile matches the actual behavior of the gas observed by astronomers.
Moreover, the parameters used in the model, such as mass and interaction strength of dark matter, do not violate any of the known constraints from primordial universe physics.
The Link with Gamma Rays
The same annihilation that generates electrons also produces positrons. These can slow down and meet electrons from the environment. When this happens, gamma rays with an energy of 511 keV are emitted.
The hypothesis, therefore, unites the two phenomena in a single explanation: light dark matter and its interactions at the galactic center.
The amount of brightness generated depends on several factors, such as how efficiently positrons combine with electrons and the exact location where these annihilations occur. These details are not fully defined yet, but indications point to a possible connection between the two signals.
New Tool to Study the Invisible
Even though the link with gamma rays still needs more evidence, researchers emphasize that the ionization rate in the CMZ can be used as a new tool to investigate dark matter.
As these particles are very light, laboratory experiments on Earth have difficulty detecting them. But observing the behavior of gas at the center of the galaxy may be an effective way to identify their presence.
Simulations also indicate that the distribution of ionization caused by dark matter would be relatively uniform throughout the CMZ. This aligns with what telescopes are observing.
Sources like the central black hole or supernova explosions tend to produce localized ionizations, not as spread out. On the other hand, a halo of dark matter distributed around the galactic center fits this profile better.
What the Future May Reveal
The study suggests that the center of the Milky Way is a promising region for revealing the nature of dark matter. With more advanced telescopes in the future, it will be possible to map both the ionization rate and the gamma ray emission more accurately.
These new observations could confirm or rule out the hypothesis that both phenomena are caused by the same source.
In the meantime, the CMZ continues to be closely monitored. Each new measurement may help to decipher one of the greatest mysteries of modern physics.
Even without definitive answers, the work reinforces a simple yet powerful idea: the universe still has many secrets. And sometimes, just looking inward, to the heart of our own galaxy, is enough to find clues about what lies beyond.
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