Sometimes, not seeing something can be just as important as detecting it. That idea is at the heart of a new study published in the Journal of Cosmology and Astroparticle Physics (JCAP). The research suggests scientists may not need to find identical signals everywhere in the universe to understand dark matter.
The study focuses on a puzzling observation. Astronomers have detected an excess of gamma radiation at the center of the Milky Way, which could be produced when dark matter particles collide and annihilate. However, similar signals have not been found in other places, such as dwarf galaxies. According to the new work, that absence does not necessarily rule out dark matter as the source.
Instead, dark matter itself may be more complex than previously thought. Rather than a single type of particle, it could consist of multiple components that behave differently depending on their environment.
The Milky Way’s Gamma-Ray Excess
Dark matter is believed to make up a large portion of the universe, yet it has never been directly observed. Scientists infer its existence from the way its gravity affects visible matter. Despite decades of research, its true nature remains unknown.
Many leading theories describe dark matter as being made of particles. In some models, when two of these particles meet, they annihilate and produce high-energy radiation such as gamma rays. Detecting this radiation is one of the main strategies scientists use to search for dark matter.
“Right now there seems to be an excess of photons coming from an approximately spherical region surrounding the disk of the Milky Way,” explains Gordan Krnjaic, a theoretical physicist at the Fermi National Accelerator Laboratory (Fermilab) in the United States and one of the study’s authors. Observations from the Fermi Gamma-ray Space Telescope have revealed this unusual glow, which could be linked to dark matter. Still, other explanations are possible, including gamma rays produced by astrophysical sources like pulsars.
To better understand the origin of this signal, scientists look beyond our galaxy. “If certain theories of dark matter are true, we should see it in every galaxy, for example in every dwarf galaxy,” explains Krnjaic.
Why Dwarf Galaxies Matter
Dwarf galaxies are small, faint systems that contain large amounts of dark matter. Because they have relatively few stars and less background radiation, they provide a cleaner setting to search for dark matter signals.
Standard particle-based models of dark matter typically describe two main possibilities for how annihilation occurs. In the simplest scenario, the likelihood of annihilation is constant and does not depend on how fast the particles are moving. If this is true, a signal seen in the Milky Way should also appear in other dark matter-rich systems, including dwarf galaxies.
In another scenario, the annihilation rate depends on particle velocity. Since dark matter particles move slowly within galaxies, this type of interaction would make annihilation extremely rare, leaving little or no detectable signal anywhere.
Under these conventional frameworks, the lack of gamma-ray emission from dwarf galaxies makes it harder to interpret the Milky Way’s gamma-ray excess as evidence of dark matter.
A Two-Component Dark Matter Model
Krnjaic and his collaborators propose a different explanation that could resolve this tension. Their model suggests dark matter may consist of two distinct types of particles rather than one.
“What we’re trying to point out in this paper is that you could have a different kind of environmental dependence, even if the annihilation probability is constant in the center of the galaxy,” explains Krnjaic. “Dark matter could straightforwardly be two different particles, and the two different particles need to find each other in order to annihilate.”
In this picture, the chance of annihilation depends not only on how often particles interact, but also on the balance between the two types of dark matter in a given system. That balance may vary from one galaxy to another. In a galaxy like the Milky Way, the two particle types could exist in similar amounts, making interactions more likely. In dwarf galaxies, one type might dominate, reducing the chance of collisions and limiting any detectable signal.
“In this way, you get very different predictions for the emission,” explains Krnjaic.
What Future Observations Could Reveal
This two-component model offers a more flexible way to interpret current observations. It allows scientists to explain why the Milky Way shows a gamma-ray signal while dwarf galaxies do not, without discarding the possibility that dark matter is responsible.
Future observations will be crucial for testing this idea. The Fermi Gamma-ray Telescope may provide more detailed data on dwarf galaxies, where measurements are still limited. Detecting gamma rays in these systems could indicate a similar mix of dark matter components. On the other hand, continued non-detection might suggest that one component is less common in those environments.
Even so, the interpretation is not straightforward. Other astrophysical factors could influence what is observed, meaning scientists will need to compare this model with a broad range of data.
The paper “dSph-obic dark matter” by Asher Berlin, Joshua Foster, Dan Hooper and Gordan Krnjaic is now available in JCAP.