  Recent observations have revealed the existence of an abundant population of
faint, low surface brightness (SB) galaxies, which appear to be numerous and
ubiquitous in nearby galaxy clusters, including the Virgo, Coma and Fornax
clusters. With median stellar masses of dwarf galaxies, these ultra-diffuse
galaxies (UDGs) have unexpectedly large sizes, corresponding to a mean SB of
$24\lesssim\langle\mu_e\rangle_r\ {\rm mag}^{-1} {\rm arcsec}^2\lesssim27$
within the effective radius. We show that the UDG population represents the
tail of galaxies formed in dwarf-sized haloes with higher-than-average angular
momentum. By adopting the standard model of disk formation -- in which the size
of galaxies is set by the spin of the halo -- we recover both the abundance of
UDGs as a function of the host cluster mass and the distribution of sizes
within the UDG population. According to this model, UDGs are not failed $L_*$
galaxies, but genuine dwarfs, and their low SB is not uniquely connected to the
harsh cluster environment. We therefore expect a correspondingly abundant
population of UDGs in the field, with likely different morphologies and
colours.

Astrophysics of Galaxies