Every day, space is filled with thousands of mysterious bursts of radio energy, enormous flashes that typically pack as much energy into a few milliseconds as the sun emits in all wavelengths in half a day. Nobody knows what causes these fast radio bursts (FRBs), but theories include such dramatic possibilities as colliding neutron stars or neutron stars being eaten by black holes. Now, a team of astronomers has witnessed the brightest such flash to date. Because it was so bright, the scientists could observe how the signal was altered by the intergalactic medium through which it traveled, much as starlight’s passage through Earth’s atmosphere makes stars twinkle. Based on how the signal dispersed and distorted, the team confirmed that the signal had been traveling for at least a billion years—meaning its source was at least a billion light-years away, they report today in Science. The team also used the flash to determine properties of the tenuous plasmas between galaxies—such as magnetism and turbulence, confirming earlier theories that it is neither highly magnetized nor turbulent. That’s important, say the scientists, because there are few other ways to study the galactic medium, which contains nearly 40% of all nondark matter in the galaxy. But because FRBs occur without warning and don’t repeat, it’s unlikely that scientists will get a similar chance any time soon. The team, which was watching a neutron star in our own galaxy, was lucky enough to be pointing its telescope in precisely the right direction.