Carlos Solis
Astrophysicists announced the discovery of an extremely dark galaxy — an object whose mass is about 99.9 percent dark matter. The galaxy emits almost no light, so it could provide crucial clues to one of modern cosmology’s biggest mysteries.
How do you spot something that gives off no light?
By definition, “dark” galaxies don’t emit light, so astronomers can’t detect them with direct imaging. The research team looked instead for the galaxy’s gravitational fingerprints on nearby stars and gas.
“Typically, to measure a galaxy’s dark-matter content, you look at how objects move,” explains Professor Francine Marlow of the University of Innsbruck, a member of the team. From those motions, astronomers can infer how much invisible mass is required to hold the orbits together.
In dark galaxies, ordinary matter is so scarce that tracking stellar motions directly becomes difficult. The team used another marker — globular clusters, which are dense groups of tens of thousands to millions of stars that remain visible at large distances.
The astronomers found a group of four globular clusters about 250 million light-years away. The clusters looked as if they were “hovering” around empty space, yet gravity kept them bound. The researchers labeled the object Dark Galaxy Candidate-2, or CDG-2.
A closer look revealed faint, diffuse light between the clusters. That light corresponds to roughly 0.1 percent of CDG-2’s mass in visible matter, which supports the interpretation that this is a dark galaxy.
Why dark galaxies matter
Dark matter builds the scaffolding that ordinary matter collects on to form galaxies. Understanding how systems with few visible stars form helps astronomers reconstruct the assembly history of galaxies.
“These faint systems, when observable, help us piece together how a galaxy assembled,” says Marlow. If ordinary matter never accreted or if star formation shut down very early, a system can end up almost entirely dark — a true dark galaxy.
Dark galaxies as a test for dark-matter models
Researchers hope CDG-2 can be used to test competing theories about the nature of dark matter. “The problem when studying dark matter in bright galaxies like the Milky Way or Andromeda is that visible matter masks signals that would reveal dark-matter properties,” says Dr. David Lee of the University of Toronto, lead author of the study.
In dark galaxies, visible matter is much reduced, so the gravitational “fingerprint” of dark matter stands out more clearly. Because of that clarity, these objects can help distinguish between different dark-matter models.
Main theoretical options
- Cold dark matter — the most widely used model — envisions heavy particles that interact weakly with electromagnetic radiation and moved slowly in the early universe.
- Warm dark matter posits lighter, faster particles that fail to bind gravitationally on the smallest scales.
- Hot dark matter originally proposed extremely light, fast particles, but simulations showed that such properties would prevent large structures like galaxies from forming, so the idea lost favor.
- “Fuzzy” dark matter describes ultralight particles that behave like waves and produce quantum effects noticeable on large scales.
- Self-interacting dark matter allows dark-matter particles to interact with one another, creating a “dark sector” of physics with its own analogs of photons or fermions.
To test these ideas, researchers run computer simulations, compare the results with real observations, and see which model best reproduces the universe. Simpler models are easier to formalize and code for simulators, but sometimes more complex variants reproduce observed behavior better.
Sylvia Plokinger of the University of Vienna cautions that mismatches between simulations and observations sometimes reflect incomplete understanding of smaller-scale processes — for example, the role of supernovae in shaping galaxies — rather than a failure of cosmology itself.
What comes next
Confirming CDG-2 and finding similar objects would open a new path for dark-matter research. The team also identified another candidate, CDG-1, which could be even darker and might be composed almost entirely of dark matter.
Confirming such an object will require powerful telescopes and very long exposures. “Even with the James Webb Space Telescope you need the most precise setup and extremely long exposures,” Lee explains, which makes securing observing time difficult. If observers get the time and confirm the object, it would become a textbook example of a dark galaxy.
After that confirmation, scientists could use these systems as natural laboratories to eliminate competing dark-matter theories and possibly get closer to answering one of physics’ oldest questions.
Based on reporting from BBC Science Focus
