Intense, brief flashes of radio light called fast radio bursts (FRBs) travel across billions of light-years to reach Earth, passing through a fog of matter along the way. The bursts' origins are unclear but may originate from highly magnetized dead stars called magnetars. The denser the fog through which FRBs travel, the more dispersed their signals become—similar to the way a prism splits white light into a rainbow of colors.