At the very heart of the Milky Way, something is continuously glowing in gamma rays. This light is special: its energy is exactly 511 thousand electronvolts. Such energy is produced when an electron meets its antimatter counterpart, a positron, and they annihilate, leaving behind a flash of light. Astronomers have been seeing this glow for over fifty years, but they still don't know exactly where the positrons come from.
The new idea is to send a balloon into the sky carrying eight thousand tiny detectors. Each detector is a piece of bismuth metal the size of a grain of rice. When a gamma-ray photon hits such a piece, the metal warms up slightly, and an ultra-sensitive film registers this heating. This allows measuring the light's energy with an accuracy of 200 electronvolts, like spotting individual grains of sand on a beach from a bird's-eye view.
Such precision will allow us to see for the first time whether the glow is uniform or consists of distinct bright spots. If there are spots, it means positrons are born in specific places — perhaps near neutron stars or in the vicinity of the black hole at the galaxy's center. If the light is uniform, the culprit might be dark matter slowly decaying throughout the center.
The balloon with detectors will ascend to an altitude where the air barely interferes with gamma rays. In a single flight, it will collect so much light that chance coincidences will be ruled out: the signal will be 35 times stronger than any noise. And then, perhaps, the half-century-old mystery will finally be solved.
To catch a gamma-ray photon with an energy of 511 keV, the detector only needs to warm up by one ten-millionth of a degree, and the instrument will notice it.