Photon underproduction crisis

The photon underproduction crisis is a cosmological discussion concerning the purported deficit between observed photons and predicted photons. The deficit, or underproduction crisis, is a theoretical problem, arising from comparing observations of ultraviolet light emitted from known populations of galaxies and quasars to theoretical predictions of the amount of ultraviolet light required to simulate the observed distribution of the hydrogen gas in the local universe in a cosmological simulation.

Source: Wikipedia — Photon underproduction crisis (CC BY-SA 4.0)