A distant gamma-ray beam helps measure the light filling the universe

High-energy photons from the blazar OP 313 interact with the diffuse light produced by stars and galaxies. Some are lost through interactions that create an electron and a positron.

By comparing the surviving radiation across energies, researchers can constrain how much background light it passed through. The study combines observations from the LST-1 and MAGIC telescopes with other measurements.

This is an indirect inference: it requires understanding the source as well as the intervening universe. The result provides limits on the background radiation rather than a photograph of every contributing galaxy. The method is compelling because attenuation, usually an obstacle to observation, becomes information about material and light spread across immense distances.