  Galaxies are surrounded by large reservoirs of gas, mostly hydrogen, fed by
inflows from the intergalactic medium and by outflows due to galactic winds.
Absorption-line measurements along the sightlines to bright and rare background
quasars indicate that this circumgalactic medium pervades far beyond the extent
of starlight in galaxies, but very little is known about the spatial
distribution of this gas. A new window into circumgalactic environments was
recently opened with the discovery of ubiquitous extended Lyman-alpha emission
from hydrogen around high-redshift galaxies, facilitated by the extraordinary
sensitivity of the MUSE instrument at the ESO Very Large Telescope. Due to the
faintness of this emission, such measurements were previously limited to
especially favourable systems or to massive statistical averaging. Here we
demonstrate that low surface brightness Lyman-alpha emission surrounding faint
galaxies at redshifts between 3 and 6 adds up to a projected sky coverage of
nearly 100%. The corresponding rate of incidence (the mean number of
Lyman-alpha emitters penetrated by any arbitrary line of sight) is well above
unity and similar to the incidence rate of high column density absorbers
frequently detected in the spectra of distant quasars. This similarity suggests
that most circumgalactic atomic hydrogen at these redshifts has now been
detected also in emission.
