Already in the early Universe, supermassive black holes with masses a billion times larger than the Sun appear to inhabit the centres of massive galaxies. As interstellar gas is accelerated in their powerful gravitational field, it emits copious amounts of radiation, outshining the entire galaxy as “quasars”. Recent observations have revealed that the first quasars are often surrounded by bright, giant nebulae. These can span up to several 100,000 light years, about ten times larger than their host galaxy. New detailed computer simulations of galaxy evolution performed at MPA have shed new light on these puzzling observations, reproducing them in striking detail. According to these new theoretical models, the observed extended nebulae can be explained as quasar light that reflects off cool neutral hydrogen clouds surrounding the quasar host galaxy. Crucially, this mechanism only works if the energy provided by the quasar is able to produce gigantic galactic winds that blow out large masses of gas from its immediate vicinity. This finding suggests that quasars shape galaxy evolution from the earliest stages of galaxy formation.