Astronomers using the Hubble Space Telescope have discovered an extremely faint galaxy that may be made almost entirely of dark matter, the mysterious substance that cannot be seen but is thought to make up most of the universe’s mass.
The newly studied object, called CDG-2, belongs to a rare class known as low-surface-brightness galaxies.
These galaxies are so dim that they are nearly invisible, containing only a small number of faint stars surrounded by vast amounts of unseen matter.
Most galaxies shine brightly because of the billions of stars they contain. In contrast, galaxies like CDG-2 are difficult to detect because they emit very little light.
Scientists believe they are dominated by dark matter, which does not reflect, emit, or absorb light, making it detectable only through its gravitational effects.
To find this hidden galaxy, researchers led by David Li from the University of Toronto used an unusual strategy.
Instead of searching for the faint glow of stars, they looked for clusters of stars called globular clusters.
These clusters are tightly packed groups of ancient stars that often orbit galaxies. Even if a galaxy itself is almost invisible, its globular clusters can still be spotted.
The team first identified possible candidates by finding groups of these clusters that appeared to be traveling together.
To confirm the discovery, they combined observations from three powerful observatories: NASA’s Hubble Space Telescope, the European Space Agency’s Euclid space telescope, and the Subaru Telescope in Hawaii.
Hubble’s sharp images revealed four globular clusters close together in the Perseus galaxy cluster, about 300 million light-years from Earth. Further analysis showed a faint glow surrounding them, indicating the presence of an underlying galaxy.
Scientists estimate that CDG-2 contains about as much visible light as six million suns, which is extremely small compared to typical galaxies. Surprisingly, the four globular clusters account for a large portion of its visible matter.
Calculations suggest that about 99 percent of the galaxy’s total mass is dark matter, making it one of the most dark-matter-dominated galaxies ever discovered.
Researchers think CDG-2 may once have contained more normal matter, such as hydrogen gas needed to form stars. However, interactions with nearby galaxies inside the dense Perseus cluster likely stripped much of this material away, leaving behind a ghostly structure held together mainly by dark matter.
Discoveries like this help scientists understand how galaxies form and evolve, and how dark matter shapes the universe.
As new space missions and large sky surveys come online in the coming years, astronomers expect to find more of these hidden galaxies. Each one could provide important clues about the invisible forces that govern cosmic structure.
