  Our statistical understanding of galaxy evolution is fundamentally driven by
objects that lie above the surface-brightness limits of current wide-area
surveys (mu ~ 23 mag arcsec^-2). While both theory and small, deep surveys have
hinted at a rich population of low-surface-brightness galaxies (LSBGs) fainter
than these limits, their formation remains poorly understood. We use
Horizon-AGN, a cosmological hydrodynamical simulation to study how LSBGs, and
in particular the population of ultra-diffuse galaxies (UDGs; mu > 24.5 mag
arcsec^-2), form and evolve over time. For M* > 10^8 MSun, LSBGs contribute 47,
7 and 6 per cent of the local number, mass and luminosity densities
respectively (~85/11/10 per cent for M* > 10^7 MSun). Today's LSBGs have
similar dark-matter fractions and angular momenta to high-surface-brightness
galaxies (HSBGs; mu < 23 mag arcsec^-2), but larger effective radii (x2.5 for
UDGs) and lower fractions of dense, star-forming gas (more than x6 less in UDGs
than HSBGs). LSBGs originate from the same progenitors as HSBGs at z > 2.
However, LSBG progenitors form stars more rapidly at early epochs. The higher
resultant rate of supernova-energy injection flattens their gas-density
profiles, which, in turn, creates shallower stellar profiles that are more
susceptible to tidal processes. After z ~ 1, tidal perturbations broaden LSBG
stellar distributions and heat their cold gas, creating the diffuse, largely
gas-poor LSBGs seen today. In clusters, ram-pressure stripping provides an
additional mechanism that assists in gas removal in LSBG progenitors. Our
results offer insights into the formation of a galaxy population that is
central to a complete understanding of galaxy evolution, and which will be a
key topic of research using new and forthcoming deep-wide surveys.
