Free Astronomy Magazine September-October 2025

SEPTEMBER-OCTOBER 2025 These new observations place stronger limits on the behavior of dark matter particles. The strange new clumps and elon- gated line of mass the team identi- fied may mean that the Bullet Cluster was produced by more than one collision of galaxy clusters bil- lions of years ago. The larger clus- ter, which now sits on the left, might have suffered a minor colli- sion before it rammed through the galaxy cluster now at right. The same larger cluster may also have experienced a violent interaction afterward, causing an additional shake up of its contents. “A more complicated scenario would lead to a huge asymmetric elongation like we see on the left,” Jee said. persuasive: “We confirmed that the intracluster light can be a reliable tracer of dark matter, even in a highly dynamic environment like the Bullet Cluster,” Cha said. If these stars are not bound to galaxies, but to the cluster’s dark matter, it might become easier to pin down more specifics about the invisible matter. Viewed as a whole, the researchers’ new measurements significantly re- fine what we know about how mass is spread throughout the Bullet Cluster. The galaxy cluster on the left has an asymmetric, elongated area of mass along the left edge of the blue region, which is a clue pointing to previous mergers in that cluster. Dark matter does not emit, reflect, or absorb light, and the team’s findings indicate that dark matter shows no signs of significant self-interaction. If dark matter did self-interact in Webb’s observations, the team would see an offset be- tween the galaxies and their respec- tive dark matter. “As the galaxy clusters collided, their gas was dragged out and left behind, which the X-rays confirm,” Finner said. Webb’s observations show that dark matter still lines up with the galaxies — and was not dragged away. Although earlier measurements with other telescopes also identified invisible mass in addition to the mass in the galaxies, it was still pos- sible that the dark matter could in- teract with itself to some degree. !