Dark dwarfs are hypothetical dark matter-powered objects that formed from the cooling of brown dwarfs, according to a team of astronomers from Durham University, the University of Hawai’i and the University of Liverpool.

What we know today about dark matter is that it exists and how it behaves — but not yet what it actually is.

Over the past fifty years, several hypotheses have been proposed, but none have yet gathered enough experimental evidence to prevail.

Among the most well-known dark matter candidates are the Weakly Interacting Massive Particles (WIMPs) — very massive particles that interact very weakly with ordinary matter: they pass through things unnoticed, don’t emit light and don’t respond to electromagnetic forces, and reveal themselves only through their gravitational effects.

This type of dark matter would be necessary for dark dwarfs to exist.

“Dark matter interacts gravitationally, so it could be captured by stars and accumulate inside them,” said University of Hawai’i Professor Jeremy Sakstein.

“If that happens, it might also interact with itself and annihilate, releasing energy that heats the star.”

Ordinary stars shine because nuclear fusion processes occur in their cores, generating large amounts of heat and energy.

Fusion happens when a star’s mass is large enough that gravitational forces compress matter toward the centre with such intensity that they trigger reactions between atomic nuclei.

This process releases a huge amount of energy, which we see as light. Dark dwarfs also emit light — but not because of nuclear fusion.

“Dark dwarfs are very low mass objects, about 8% of the Sun’s mass,” Professor Sakstein said.

“Such a small mass is not sufficient to trigger fusion reactions.”

“For this reason, such objects — although very common in the Universe — usually only emit a faint light and are known to scientists as brown dwarfs.

However, if brown dwarfs are located in regions where dark matter is particularly abundant — such as the centre of our Milky Way Galaxy — they can transform into something else.

“These objects collect the dark matter that helps them become a dark dwarf,” Professor Sakstein said.

“The more dark matter you have around, the more you can capture.”

“And, the more dark matter ends up inside the star, the more energy will be produced through its annihilation.”

“For dark dwarfs to exist, dark matter has to be made of WIMPs, or any heavy particle that interacts with itself so strongly to produce visible matter.”

“Other candidates proposed to explain dark matter — such as axions, fuzzy ultralight particles, or sterile neutrinos — are all too light to produce the expected effect in these objects.”

“Only massive particles, capable of interacting with each other and annihilating into visible energy, could power a dark dwarf.”

This entire hypothesis, however, would have little value if there weren’t a concrete way to identify a dark dwarf.

For this reason, Professor Sakstein and colleagues propose a distinctive marker.

“There were a few markers, but we suggest lithium-7 because it would really be a unique effect,” Professor Sakstein said.

“Lithium-7 burns very easily and is quickly consumed in ordinary stars.”

“So if you were able to find an object which looked like a dark dwarf, you could look for the presence of this lithium because it wouldn’t be there if it was a brown dwarf or a similar object.”

The team’s work appears in the Journal of Cosmology and Astroparticle Physics.

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Djuna Croon et al. 2025. Dark dwarfs: dark matter-powered sub-stellar objects awaiting discovery at the Galactic center. JCAP 07: 019; doi: 10.1088/1475-7516/2025/07/019