Gamma-ray astronomy, a new window for viewing the universe, is providing important opportunities for testing current understanding of the evolution of stars and galaxies. Recently, using a gamma-ray telescope in orbit around the Earth, scientists discovered a cloud of positrons—the antimatter counterpart of the electron—in a part of the Milky Way Galaxy where there are no known sources of positrons. This discovery is causing scientists to rethink the theories for the production and transport of positrons in the Milky Way Galaxy, and may provide clues to improve understanding of the creation of elements in stars and the history of stellar evolution in the central region of the Galaxy.