Dark matter may come in multiple forms, new model suggests – Astronomy Now

Astronomers may not need to see the same dark matter signal everywhere in the Universe to confirm its existence. A new theoretical study suggests that dark matter could consist of more than one type of particle, potentially resolving a long-standing observational puzzle.

Dark matter itself is inferred because the visible contents of the Universe (stars, gas and dust) cannot account for the gravitational effects that are observed. Galaxies rotate too quickly to be held together by their luminous matter alone, galaxy clusters would disperse without additional unseen mass, and patterns in the cosmic microwave background require far more matter than we can detect directly. These all point to a substantial component of invisible matter that interacts gravitationally but does not emit, absorb or reflect light, hence the name ‘dark matter’.

The work, published in the Journal of Cosmology and Astroparticle Physics, revisits one of the most debated hints of dark matter: an excess of gamma rays observed at the centre of the Milky Way by NASA’s Fermi Gamma-ray Space Telescope.  

This emission has been interpreted by some researchers as a possible signal of dark matter particles annihilating each other. However, the same signal has not been convincingly detected in smaller, dark matter–rich systems such as dwarf galaxies. This discrepancy has cast doubt on the interpretation.

“If certain theories of dark matter are true, we should see it in every galaxy,” said Gordan Krnjaic, one of the study’s authors.  

Dwarf galaxies are considered prime targets in the search for dark matter. They contain large amounts of the unseen material but relatively few stars, meaning there is little astrophysical ‘background noise’ to obscure potential signals. In conventional models, if dark matter annihilation produces gamma rays in the Milky Way, similar emissions should also be detectable in these quieter environments.

The absence of such a signal has therefore been a significant challenge. Either the Milky Way’s gamma-ray excess is not caused by dark matter (perhaps instead arising from a population of unresolved pulsars) or the underlying theory is incomplete.

The new study offers a third possibility.

Instead of assuming dark matter is made of a single particle species, the researchers propose that it may consist of two distinct types of particle. Crucially, these particles would only annihilate when they encounter each other — not when they meet identical counterparts.

“What we are trying to point out is that you could have a different kind of environmental dependence,” said 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 scenario, the strength of any gamma-ray signal depends not only on the overall amount of dark matter present, but also on the relative abundance of the two particle types. If both components are present in similar proportions, as might be the case in the Milky Way, annihilations could occur frequently enough to produce a detectable signal.

By contrast, if one component dominates in dwarf galaxies, encounters between the two species would be rare, suppressing the gamma-ray emission even in regions rich in dark matter.

This ‘two-state’ model therefore allows for a gamma-ray excess in the Milky Way while remaining consistent with the lack of similar detections in dwarf galaxies. It represents a more flexible alternative to standard scenarios, in which the annihilation rate is either constant or strongly dependent on particle velocity.

The idea does not yet resolve the mystery, but it reframes how astronomers interpret both detections and non-detections. In particular, it suggests that the absence of a signal in certain environments may not be decisive evidence against a dark matter origin.

Future observations will be key. More sensitive measurements of dwarf galaxies with the Fermi telescope, or its successors, could reveal faint gamma-ray emissions or confirm their absence with greater confidence. Either outcome would place tighter constraints on the proposed model.

For now, the study highlights how much remains unknown about dark matter, which is thought to make up around 85 per cent of the Universe’s matter but has never been directly detected.

Rather than a single, simple particle, it may prove to be a more complex mixture that reveals itself only under the right cosmic conditions. Or astronomers are chasing illusions and the real reason for the cosmic discrepancies is that we do not fully understand gravity.

Read the original paper here.