Strange events are happening in the center of the Milky Way — and this is a hint of a new kind of dark matter

Published by Kateryna Danshyna

11.03.2025 13:17

Центр Чумацького Шляху. Зображення: NASA/JPL-Caltech, CC BY-NC

Scientists have probably found a new kind of dark matter — and it is hiding in the very center of the Milky Way. Compared to other candidates, this phenomenon is not only lighter, but also has the ability to self-destruct.

Miracles in the center of a nearby galaxy

When two particles of this potential new kind of dark matter meet, they destroy each other and create a negatively charged electron and its positively charged equivalent, a positron — this process, known as ionization, provides the energy needed to tear electrons away from neutral atoms in the dense gas at the center of the Milky Way. In fact, this may be a perfectly good explanation for why there is so much ionized gas in the heart of our galaxy (a place known as the Central Molecular Zone or CMZ).

«We hypothesize that dark matter lighter than a proton may be responsible for the unusual effect observed in the center of the Milky Way. Unlike most candidates, which are often studied because of their gravitational effects, this type of dark matter could manifest itself through an ionizing gas, essentially stripping electrons from atoms in the CMZ»,” explains team leader and King’s College London research fellow Shyam Balaji.

Dark Matter — is a hypothetical component of the Universe that, if we exclude dark energy, accounts for 80% of its total composition. Despite this omnipresence, scientists cannot see dark matter, but detect its presence solely by its gravitational influence on visible matter and relic radiation.

A timeline of the Universe from the Big Bang (left) to the present (right). For most of this time, dark matter has governed the cosmos, forming galaxies and galaxy clusters; dark energy dominates the future, pushing galaxies further and further apart. Source: NASA/WMAP

Currently, the main candidates for dark matter are axions and axion-like particles, but Balaji and colleagues have ruled them out as culprits in the phenomenon occurring in the center of the Milky Way.

«Most axion models do not predict significant annihilationAnnihilation — the interaction of elementary particles and antiparticles that results in their transformation into other forms of matter, e.g., the transformation of an electron and a positron into a pair of photons. into electron-positron pairs, as our proposed dark matter does», Balaji says. «It has a mass of one billion electrons and self-annihilates into electrons and positrons, which sets it apart from the others. This species has a direct impact on the interstellar medium, creating signs in the form of additional ionization, which is not usually expected from» axions.

Dark Matter — the main candidate

Under conditions of high CMZ density, positrons cannot travel far without first interacting with neighboring hydrogen molecules and capturing their electrons, so the process looks particularly efficient in this region.

«The biggest problem this model helps to solve is the excess ionization in the CMZ», Balaji says. «Cosmic rays, the usual culprits of gas ionization, don’t seem to be strong enough to explain the high levels of ionization we observe».

Cosmic rays — are high-energy elementary particles that travel at speeds close to light. But the ionization signals from CMZ, according to the team, indicate a slower moving source that is lighter than many other dark matter candidates. In addition, if cosmic rays were the ionizing gas in the CMZ, there should be associated gamma-ray emission, but researchers in the region have not seen this so far.

The central molecular zone in the Milky Way contains many mysteries — and a new study explains them by the presence of a new kind of dark matter. Source: ESA/C.Carreau

In addition, the dark matter annihilation model could also explain the characteristic light emission from the CMZ, which results from positively charged positrons and negatively charged electrons colliding and combining into a state called positronium, which then quickly decays into X-rays, light with slightly lower energy than gamma rays.

«The numbers match much better than we expected. Often, explanations of dark matter run into problems because they predict signals that should have already been detected by telescopes», — Balaji adds. «But in this case, the ionization rate produced by this dark matter fits perfectly with known constraints and does not contradict observations of gamma rays and the cosmic microwave background»

Another theory

Obviously, the new dark matter candidate is at the very beginning of its theoretical life and does not even have an official name yet. The axion explanation, by comparison, has been around since 1978, when it was first proposed by theoretical physicists Frank Wilczek and Steven Weinberg.

«We need more precise measurements of ionization in the CMZ; if we can map the ionization more accurately, we can see if it matches the expected distribution of dark matter», Balaji concludes. «If we rule out other potential sources of ionization, the dark matter hypothesis becomes more convincing».

NASA’s upcoming COSI (Compton Spectrometer and Imager) space telescope may help in the search for further evidence of the connection between annihilating dark matter and strange emissions from the CMZ — it was planned to be launched in 2027, but due to the restructuring of the agency with the arrival of the new American government, these plans may still change.

The study was published in the journal Physical Review Letters (via Space.com).