Free Astronomy Magazine May-June 2026

MAY-JUNE 2026 T he low-surface-brightness galaxy CDG-2, shown in this image from the NASA/ESA Hubble Space Telescope, is dominated by dark matter and con- tains only a sparse scattering of stars. This galaxy is nearly invisible, but by using advanced statistical techniques, scientists identified it by searching for tight groupings of stars called globular clusters. At left, the white box marks the area that was examined. At right is a magnified view of that area. The cir- cle marked with a dashed orange boundary indicates the location of the dark- matter dominated galaxy. Within the dashed circle are four globular clusters outlined by small, blue circles. Several background galaxies also appear within the orange circle, but these are not related to the galaxy CDG-2. [NASA, ESA, D. Li (Utoronto), Image Processing: J. DePasquale (STScI)] galaxy behind the globular clusters for the first time,” says Francine Marleau from the Institute for Astro- and Particle Physics at the University of Innsbruck in Austria. “The Euclid images of the Perseus cluster demonstrate the mission’s unique capability to detect new low-surface-brightness galaxies, in- cluding extremely faint ones, while also revealing their globular clus- ters, nuclear star clusters, internal structures, and surrounding envi- ronments.” Globular clusters possess immense stellar density and are gravitationally tightly bound. This makes the clusters more resistant to gravitational tidal disruption, and therefore reliable tracers of such ghostly galaxies. matter to enable star formation (primarily hydrogen gas) was likely stripped away by gravitational in- teractions with other galaxies inside the Perseus cluster. “The Euclid data clearly confirm the presence of the extremely faint, dif- fuse light of CDG-2, revealing the Preliminary analysis suggests CDG-2 has the luminosity of roughly 1 mil- lion Sun-like stars, with the globular clusters accounting for 16% of its visible content. Remarkably, 99% of its mass, which includes both visible matter and dark matter, appears to be dark matter. Much of its normal !