The clue is a gamma ray with a precise energy
A positron is an electron’s antimatter counterpart: it has the same mass as an electron but the opposite electric charge. When the two meet, they annihilate. In a common outcome, their mass becomes two gamma-ray photons, each carrying 511,000 electron volts of energy—the energy equivalent of an electron’s mass.
That precise energy gives astronomers a way to identify the process. Instruments sensitive to gamma rays can measure the energies of incoming photons and look for a distinctive signal from the Milky Way’s inner regions. The photons travel to us without needing to be visible light, so the signal can reveal activity hidden behind clouds of dust that block many ordinary telescopes.
The signal does not identify its source
The difficult question is where the positrons come from. Some are produced by radioactive atoms made in energetic events such as stellar explosions. Other proposed sources include activity around the galaxy’s central black hole and the possible annihilation of dark-matter particles. Seeing the annihilation signature is not the same as knowing what created the positrons.
There is another complication: positrons can travel away from where they were born before they meet electrons. The observed glow therefore need not map neatly onto the objects that produced them. Astronomers compare the signal’s shape and spectrum with predictions for different sources, but the central region’s prominent emission has not yielded a single settled explanation.
A glow, not a blast wave
Calling the event an “explosion” can suggest one enormous detonation. In reality, the signal is the accumulated radiation from many particle-by-particle encounters, spread across a region and observed over time. The striking implication is that a map of invisible gamma rays can trace the last moments of antimatter—even while the origin of that antimatter remains an open puzzle.
