Astronomers may have solved one of the most vexing mysteries in our Milky Way: the origin of a giant cloud of antimatter surrounding the galactic center. Integral found that the cloud extends farther on the western side of the galactic center than it does on the eastern side. This imbalance matches the distribution of a population of binary star systems that contain black holes or neutron stars, strongly suggesting that these binaries are churning out at least half of the antimatter, and perhaps all of it.