For generations, physicists have known that most of the universe is made of something they cannot see — dark matter, inferred from the way galaxies move but never directly caught. Now, a detector built for exactly this purpose has registered an anomalous signal that does not behave like noise, and the scientific world is paying close attention. It is not yet confirmation, but it may be the first genuine handhold on one of the deepest mysteries the cosmos has posed to human inquiry.