A cosmic engine is hurling strange particles at Earth—and new observations, published today in the journal Science, are complicating the hunt for the culprit.

In 2008, the space-based instrument PAMELA detected an overabundance of particles called positrons in Earth’s atmosphere. These particles are a form of antimatter, a substance that is the opposite of normal matter. When an antimatter positron encounters its opposite particle, the two can annihilate one another and vanish in a tiny puff of energy that often includes gamma-rays, which scientists can detect. (Read about the first antimatter found orbiting Earth.)

Figuring out the source of this weird positron excess is exciting, because it helps scientists understand the highest-energy phenomena in the nearby universe, which in