Astronomers have just identified what appears to be a cosmic anomaly: a faint galaxy with so few visible stars that, according to calculations, as much as 99.9 percent of its mass is dark matter. The remaining 0.1 percent is conventional matter.
This galaxy, located about 300 million light-years away, is practically invisible. Only four globular clusters, small concentrations of stars that look like isolated neighborhoods in the middle of the void, stand out. For years, these star collections in the Perseus cluster were considered independent objects.
Now, after an exhaustive analysis, a study published in The Astrophysical Journal Letters presents solid evidence that these globular clusters are part of the same galaxy dominated by dark matter. Tentatively named CDG-2 (Candidate Dark Galaxy-2), it is the first galaxy to be detected only by its brightest fragments.
The authors pooled data from the Hubble, Euclid, and Subaru telescopes, three of the most powerful observatories available. The combined readings reveal an extremely faint glow around the four globular clusters. This residual light is a clear sign of an underlying galaxy so dim that the three telescopes missed it on their own.
More Than Meets the Eye
Preliminary analysis indicates that CDG-2 has a total luminosity equivalent to about 6 million suns, with the four globular clusters contributing about 16 percent of that brightness, an unusually large share. This distribution suggests that, despite its low light, the galaxy is a gravitationally bound system, implying a particularly dense dark matter halo. Astronomers estimate that this invisible structure accounts for between 99.94 to 99.98 percent of CDG-2’s total mass.
According to current models, dark matter constitutes roughly 27 percent of the universe’s total energy density and about 85 percent of its matter. Although the exact nature of what makes up dark matter is still unclear, because it neither emits nor reflects light, scientists infer its existence from its gravitational effects on radiation, visible matter, and the large-scale structure of the cosmos.
Dark matter is so pervasive throughout galaxies that its presence explains the stability and motion of stars in systems such as the Milky Way. For example, current models indicate that our galaxy is embedded in a halo composed of about 90 percent dark matter.
However, the case of CDG-2 is extreme: a galaxy with almost no stars, surrounded almost entirely by an invisible halo. These types of systems, so-called “dark galaxies,” are beginning to appear in astronomical records. Beyond their rarity, scientists value them because they serve as natural laboratories for exploring the nature of dark matter and testing current models of galaxy formation.
This story originally appeared on WIRED en Español and has been translated from Spanish.
