The universe hides its secrets in strange ways. Sometimes, what scientists don’t see matters more than what they do.
A new study suggests a bold idea. Dark matter may not be just one particle. It could be made of two types working together.
This idea may explain why a mysterious signal appears in the Milky Way but not in smaller galaxies.
The research is changing how scientists think about dark matter’s behavior across the universe.
The mystery of dark matter
Dark matter forms a large part of the universe, yet no one has seen it directly. Scientists understand its presence through gravity.
Galaxies move in ways that visible matter cannot explain, which means something invisible adds extra mass.
Many theories propose that dark matter consists of tiny particles. When these particles collide, they may destroy each other and release energy in the form of gamma rays. Telescopes like the Fermi Gamma-ray Space Telescope search for these signals.
Scientists expected to see similar signals in many galaxies. However, observations tell a different story.
The Milky Way’s strange glow
Astronomers have found a strong gamma-ray signal near the center of the Milky Way. This signal forms a bright, round glow. Some researchers think dark matter causes it.
“Right now there seems to be an excess of photons coming from an approximately spherical region surrounding the disk of the Milky Way,” explained Gordan Krnjaic from the Fermi National Accelerator Laboratory.
This signal matches what scientists expect from dark matter particles with a specific mass. However, another explanation exists. A large group of objects called pulsars could also produce similar radiation.
The mystery deepens when scientists look beyond our galaxy.
Why dwarf galaxies confuse scientists
Dwarf galaxies are small and faint, but they contain large amounts of dark matter. These galaxies should show similar gamma-ray signals if dark matter behaves the same everywhere.
“If certain theories of dark matter are true, we should see it in every galaxy, for example in every dwarf galaxy,” said Krnjaic.
However, scientists do not see this signal in dwarf galaxies. This absence creates a big challenge. In simple models, dark matter interactions should produce signals in both large and small galaxies.
This mismatch suggests that dark matter may not be as simple as once thought.
Limits of older ideas
Traditional models assume dark matter consists of one type of particle. These particles interact in predictable ways. In some models, the interaction rate stays constant. In others, it depends on how fast particles move.
Neither idea fully explains the observations. The Milky Way shows a strong signal, while dwarf galaxies show none.
Scientists expected dwarf galaxies to produce much weaker signals than the Milky Way, but still detectable with better tools.
Since scientists do not detect those signals, something seems missing in current theories.
Two forms of dark matter
Researchers propose that dark matter may consist of two types of particles instead of one. These two types must meet each other to produce gamma rays.
“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.”
The study explains that these particles exist in two states, a lighter one and a slightly heavier excited one. The heavier one forms only under certain conditions.
How energy changes everything
The key difference between galaxies lies in energy. Dark matter particles move faster in large galaxies like the Milky Way. In smaller dwarf galaxies, these particles move much slower.
The study shows that particles need enough energy to switch into the heavier state before they can interact. In the Milky Way, particles have enough energy for this change. In dwarf galaxies, they do not.
This process explains why signals appear in one place but not another.
“In this way, you get very different predictions for the emission,” said Krnjaic. This simple idea solves a long-standing puzzle without rejecting dark matter as the source.
A changing dark matter population
The study also describes how dark matter changes over time. In the early universe, both particle types existed. Later, the heavier particles became rare because they converted into lighter ones.
In large galaxies, collisions can recreate the heavier particles. This process allows signals to appear again. In dwarf galaxies, this recreation does not happen because the particles lack enough energy.
This means each galaxy can have a different mix of dark matter particles.
What future observations may show
Scientists will test this idea using better data. The Fermi Gamma ray Space Telescope continues to observe galaxies. Future missions and telescopes may provide clearer answers.
If scientists detect gamma rays in dwarf galaxies later, it may mean both particle types exist there. If signals remain missing, it may confirm that dark matter behaves differently depending on the environment.
The new model suggests that dark matter may hide its clues unevenly across space. By studying both what is seen and what is missing, scientists move closer to solving one of the biggest mysteries in science.
The study is published in the Journal of Cosmology and Astroparticle Physics.
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