Scientists may have unlocked a new way to understand dark matter by proposing that it exists in two distinct forms. A new study suggests these two forms behave very differently depending on where they are in the universe, which could explain why researchers have struggled to detect consistent signals from different regions of space.
The study, published in the Journal of Cosmology and Astroparticle Physics, was led by Asher Berlin of the Fermi National Accelerator Laboratory in the United States. Berlin and his colleagues built a model in which dark matter has two states, a lighter one and a slightly heavier one.
In the early universe, both states existed together. Over time, the heavier state got almost completely wiped out through collisions that converted it back into the lighter state. The universe was eventually left with dark matter made up almost entirely of the lighter form.
Dark matter in two forms could explain missing signals
In regions where dark matter moves fast enough, such as the halo surrounding the Milky Way and in large galaxy clusters, the lighter particles carry enough energy to transform back into the heavier state.
Once that happens, the two states collide with each other and release gamma rays, a form of high-energy light that telescopes can potentially detect. In dwarf galaxies, however, dark matter moves far too slowly for that transformation to happen.
Without the heavier state being regenerated, the two states never meet, and no detectable signal is produced. This creates a clear divide between what researchers should expect to see in different environments.
That divide is at the heart of a long-running scientific debate. For years, telescopes have detected an unexplained glow of gamma rays coming from the center of the Milky Way.
Why dwarf galaxies may stay quiet despite Milky Way glow
Some researchers believe this glow is a sign of dark matter particles, estimated to weigh around 50 billion electron volts, colliding and destroying each other. Others argue the glow comes from a large population of rapidly spinning dead stars.
Scientists have tried to settle this debate by looking for matching signals in dwarf galaxies, which are cleaner environments with fewer background sources of light. The absence of such signals has been used to argue against a dark matter explanation. But Berlin and his team say that argument no longer holds if their model is correct.
Under this framework, dwarf galaxies should show no signal, regardless of what is happening at the Galactic Center. Researchers say upcoming tools, including the Rubin Observatory, which will help discover new dwarf galaxies, and the proposed Advanced Particle-astrophysics Telescope, could directly test this idea in the coming years.
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