  Recent observations have highlighted a significant population of faint but
large (r_eff>1.5 kpc) galaxies in the Coma cluster. The origin of these Ultra
Diffuse Galaxies (UDGs) remains puzzling, as the interpretation of the
observational results has been hindered by the subjective selection of UDGs,
and the limited study of only the Coma (and some examples in the Virgo-)
cluster. We extend the study of UDGs using 8 clusters in the redshift range
0.044<z<0.063 with deep g- and r-band imaging data taken with MegaCam at the
CFHT. We describe an automatic selection pipeline for quantitative
identification, tested for completeness using image simulations of these
galaxies. We find that the abundance of the UDGs we can detect increases with
cluster mass, reaching ~200 in typical haloes of M200~10^15 Msun. The cluster
UDGs have colours consistent with the cluster red sequence, and have a steep
size distribution that declines as n~r_eff^-3.4. Their radial distribution is
significantly steeper than NFW in the outskirts, and is significantly shallower
in the inner parts. They follow the same radial distribution as the more
massive quiescent galaxies in the clusters, except within the core region of
r<0.15XR200 (or <300 kpc). Within this region the number density of UDGs drops
and is consistent with zero. These diffuse galaxies can only resist tidal
forces down to this cluster-centric distance if they are highly centrally
dark-matter dominated. The observation that the radial distribution of more
compact dwarf galaxies (r_eff<1.0 kpc) with similar luminosities follows the
same distribution as the UDGs, but exist down to a smaller distance of 100kpc
from the cluster centres, indicates that they may have similarly massive
sub-haloes as the UDGs. Although several scenarios can give rise to the UDG
population, our results point to differences in the formation history as the
most plausible explanation.
