Just left of the cluster’s central glow, a faint vertical arc traces the light of a galaxy so distant it would be invisible if not for the cosmic magnifying glass of CL0016+1609. This ghostly curve, captured by the Hubble Space Telescope, is the warped image of a galaxy billions of light-years away, its light bent by the immense gravity of merging galaxy clusters. Known also as MACS J0018.5+1626, CL0016+1609 is not just a dense swarm of galaxies—it’s a dynamic collision in motion, one of the most intensively studied clusters for its X-ray and radio emissions. When X-ray observations revealed it was actually two clusters merging along our line of sight, scientists turned to Hubble to understand how such colossal structures shape the architecture of the universe.

To map the invisible, researchers used Hubble’s Advanced Camera for Surveys to detect gravitational lensing—the bending of light caused by dark matter. Though dark matter itself remains unseen, its presence is etched across the image in the stretched and curved arcs of background galaxies. These distortions allow astronomers to calculate where dark matter concentrates within the cluster, offering clues about how it behaves during high-speed mergers. The observations were further enriched by data from Hubble’s Wide Field Camera 3, part of the Reionization Lensing Cluster Survey (RELICS), a program designed to find the earliest galaxies by using massive clusters as natural telescopes.

Among the findings, RELICS identified approximately 300 high-redshift candidate galaxies lensed by clusters like CL0016+1609—galaxies so distant they offer a glimpse into the universe’s first billion years. One such arc appears just above and to the right of the cluster’s central galaxies, a brighter but shorter streak that, like its fainter companion, carries encoded within it the story of cosmic dawn. These discoveries don’t just illuminate the far reaches of space—they help refine our understanding of how structure formed in the early universe, with galaxy clusters acting as both landmarks and lenses.

As telescopes grow more powerful, the data from Hubble’s deep observations continue to serve as a foundation for new discoveries. The arcs and smears of light in this image are more than beautiful—they are measurements, clues, and signposts, guiding astronomers toward a clearer picture of how the universe evolved from chaos to cosmos.