What's creating these unusual signals? And why does this one repeat itself?

One of the universe's deep mysteries just got a lot stranger. Astrophysicists have discovered a clue that could help explain why, every once in a while, superfast bursts of radio waves flash across Earth from deep space. But the clue — a repeating 16-day pattern in one of the bursts, undermines one of the most popular theories for where the bursts are coming from.

Fast radio bursts (FRBs) have likely happened for billions of years. But humans only discovered them in 2007, and have detected only a few dozen of them since. And in June 2019, astronomers finally tracked an FRB to its home galaxy.

But no one knows what causes them. Because these bursts are so rare, unusual and bright — considering that they're visible from billions of light-years across space — physicists have tended to assume they come from a cataclysmic event, such as the collision of stars.