An international team of astronomers has solved one of the longstanding cosmic mysteries by uncovering direct evidence of a massive, long-lost object that collided with the Perseus cluster. Using high-resolution data from the Subaru Telescope, the researchers successfully traced the remnant of this ancient merger through the dark matter distribution.
Galaxy clusters, composed of thousands of galaxies bound together by gravity, are among the most massive structures in the universe. They grow through energetic mergers—some of the most powerful events since the Big Bang.
Located about 240 million light-years from Earth, the Perseus cluster has a mass equivalent to 600 trillion suns (called solar masses). For decades, astronomers believed it had long since settled into a stable, post-merger state. Its apparent lack of clear merger signatures earned it the reputation of being the “textbook example” of a relaxed cluster.
However, advances in observational techniques have allowed researchers to peer deeper into its structure, uncovering subtle yet compelling evidence of past disruption. This raises a fundamental mystery: If there are signs of a collision, where is the object that collided with it?
To solve the mystery, the team analyzed archival data from Hyper Suprime-Cam on the Subaru Telescope. Gravitational lensing—a phenomenon where massive objects bend the light from background galaxies—served as a powerful tool to map the invisible dark matter.
Through this technique, the researchers identified a massive clump of dark matter, weighing approximately 200 trillion solar masses, located about 1.4 million light-years west of the cluster core. Remarkably, this structure is connected to the core of the Perseus cluster by a faint but statistically significant “dark matter bridge,” providing direct evidence of past gravitational interaction between them.
Numerical simulations conducted by the team suggest that this dark matter substructure collided with the Perseus cluster roughly five billion years ago. The remnants of that collision still shape the present-day structure of the cluster.
“This is the missing piece we’ve been looking for,” says Dr. James Jee, corresponding author of the study published in Nature Astronomy. “All the odd shapes and swirling gas observed in the Perseus cluster now make sense within the context of a major merger.”
“It took courage to challenge the prevailing consensus, but the simulation results from our collaborators and recent observations from the Euclid and XRISM space telescopes strongly support our findings,” continues Dr. HyeongHan Kim, the study’s first author.
“This breakthrough was made possible by combining deep imaging data from the Subaru Telescope with advanced gravitational lensing techniques we developed—demonstrating the power of lensing to unveil the hidden dynamics of the universe’s most massive structures,” says Dr. Jee.
Reference: Kim HyeongHan et al, Direct evidence of a major merger in the Perseus cluster, Nature Astronomy (2025). DOI: 10.1038/s41550-025-02530-w
