Astronomers thought the Perseus cluster was a massive-but-stable grouping of galaxies, until they found hints of a collision with another cluster — but no one had identified a cosmic interloper. Now, using a method called weak gravitational lensing, scientists think they've finally found the hidden intruder.
The Perseus galaxy cluster is one of the most massive structures in the known universe. Bearing the name of its host constellation, it's a vast grouping of thousands of galaxies spread across 11.6 million light years, roughly 100 times the diameter of the Milky Way.
