An absence or near-absence of ultraviolet light in the spectra of very distant quasars with redshifts around 6 and greater. It is caused by the presence of neutral hydrogen in the early Universe, which strongly absorbs ultraviolet photons at wavelengths shorter than Lyman-α‎. There is a similar trough due to the Lyman-α‎ line of singly ionized helium at redshifts of around 3. The trough is named after the American astrophysicists James Edward Gunn (1938– ) and Bruce Alrick Peterson (1941– ), who predicted its existence in 1965.