Sometimes not seeing something can be just as important as detecting it. This idea is at the heart of a new study published in the journal Journal of Cosmology and Astroparticle Physics (JCAP). The study suggests that scientists may not need to find the same signal everywhere in the universe to understand dark matter.
This study focuses on puzzling observations. Astronomers have detected an excess of gamma rays at the center of the Milky Way. This can be created when dark matter particles collide and annihilate. However, similar signals have not been found elsewhere, such as in dwarf galaxies. According to the new study, its absence does not necessarily exclude the possibility that dark matter is the source.
In fact, dark matter itself may be more complex than previously thought. Rather than being a single type of particle, it may consist of multiple components that behave differently depending on the environment.
Gamma ray excess in the Milky Way galaxy
Dark matter is thought to make up most of the universe, but it has never been directly observed. Scientists infer its existence based on how its gravity affects visible matter. Despite decades of research, its true nature remains unknown.
Many of the leading theories explain that dark matter is made of particles. In some models, when two of these particles meet, they annihilate, producing high-energy radiation such as gamma rays. Detecting this radiation is one of the main strategies scientists use to search for dark matter.
“Currently, it appears that an excess of photons is being emitted from a roughly spherical region surrounding the Milky Way’s disk,” explains Gordan Krunjaic, a theoretical physicist at the Fermi National Accelerator Laboratory in the United States and one of the study’s authors. Observations with the Fermi Gamma-ray Space Telescope revealed this unusual glow that may be related to dark matter. Still, other explanations are possible, such as gamma rays produced by astrophysical sources such as pulsars.
To better understand the origins of this signal, scientists are looking beyond the galaxy. “If certain theories about dark matter are true, we should see dark matter in all galaxies, for example in all dwarf galaxies,” Krnjaic explains.
Why dwarf galaxies are important
Dwarf galaxies are small, dark systems containing large amounts of dark matter. Relatively few stars and less background radiation provide a cleaner setting for searching for dark matter signals.
Standard particle-based models of dark matter typically explain two main possibilities for how annihilation occurs. In the simplest scenario, the probability of annihilation is constant and does not depend on the speed at which the particle moves. If this is true, the signals seen in the Milky Way should also appear in other dark matter-rich systems, including dwarf galaxies.
In another scenario, the extinction rate depends on the velocity of the particles. Because dark matter particles move slowly within galaxies, this type of interaction makes their annihilations extremely rare, leaving little or no detectable signal anywhere.
Under these traditional frameworks, the absence of gamma-ray emission from dwarf galaxies makes it difficult to interpret the Milky Way’s gamma-ray excess as evidence for dark matter.
Two-component dark matter model
Krunjajic and his colleagues propose an alternative explanation that could resolve this tension. Their model suggests that dark matter may be made up of not one, but two different types of particles.
“What we try to point out in this paper is that even if the probability of extinction is constant at the center of a galaxy, there can be different kinds of environmental dependence,” Krunjaic explains. “Dark matter could simply be two different particles, and for it to annihilate, the two different particles would have to find each other.”
In this picture, the probability of annihilation depends not only on how often the particles interact, but also on the balance between the two types of dark matter in a given system. That balance can vary from galaxy to galaxy. In galaxies like the Milky Way, the two types of particles can exist in similar amounts, making interactions more likely. In dwarf galaxies, one type may predominate, reducing the likelihood of collisions and limiting the detectable signal.
“This way we get very different predictions about the release,” Krunjajic explains.
What future observations will reveal
This two-factor model provides a more flexible way to interpret the current observations. This allows scientists to explain why the Milky Way shows a gamma-ray signal while dwarf galaxies do not, without leaving out the possibility that dark matter is involved.
Future observations will be critical to verify this idea. The Fermi gamma-ray telescope could provide more detailed data on dwarf galaxies, for which measurements are still limited. Detection of gamma rays in these systems may indicate a similar mixture of dark matter components. On the other hand, if it is consistently not detected, it may suggest that one component is not common in those environments.
However, the interpretation is not simple. Scientists need to compare this model to a wide range of data, as other astrophysical factors can influence observations.
The paper “dSph-obic dark matter” by Asher Berlin, Joshua Foster, Dan Hooper, and Gordan Krnjaic is currently JCAP.
