Free Astronomy Magazine May-June 2026

MAY-JUNE 2026 by NASA/ESA Bethany Downer I n the vast tapestry of the Uni- verse, most galaxies shine brightly across cosmic time and space. Yet a rare class of galaxies remains nearly invisible — low-surface- brightness galaxies dominated by dark matter and containing only a sparse scattering of faint stars. One such elusive object, dubbed CDG-2, may be among the most heavily dark matter-dominated galaxies ever discovered. Dark mat- ter is an invisible form of matter that does not reflect, emit, or ab- sorb light. The science paper detail- ing this finding was published in The Astrophysical Journal Letters . Detecting such faint galaxies is ex- traordinarily difficult. Using ad- vanced statistical techniques, Da- vid Li of the University of Toronto, Canada, and his team identified 10 previously confirmed low-surface- brightness galaxies and two addi- tional dark galaxy candidates by searching for tight groupings of globular clusters — compact, spher- ical star groups typically found or- biting normal galaxies. These clus- ters can signal the presence of a faint, hidden stellar population. To confirm one of the dark galaxy candidates, astronomers employed a trio of observatories: the Hubble Space Telescope, ESA’s Euclid space observatory, and the ground-based Subaru Telescope in Hawaii. Hubble’s high-resolution imaging revealed a close collection of four globular clusters in the Perseus gal- axy cluster, 300 million light-years away. Follow-up studies using Hubble, Eu- clid, and Subaru data then revealed a faint, diffuse glow surrounding the star clusters — strong evidence of an underlying galaxy. “This is the first galaxy detected solely through its globular cluster population,” said Li. “Under conser- vative assumptions, the four clusters represent the entire globular cluster population of CDG-2.” CDG-2 may be composed of 99% dark matter