  Galaxy surveys have shown that luminous galaxies are mainly distributed in
large filaments and galaxy clusters. The remaining large volumes are virtually
devoid of luminous galaxies. This is in concordance with the formation of the
large-scale structure in Universe as derived from cosmological simulations.
However, the numerical results indicate that cosmological voids are abundantly
populated with dark matter haloes which may in principle host dwarf galaxies.
Observational efforts have in contrast revealed, that voids are apparently
devoid of dwarf galaxies. We investigate the formation of dwarf galaxies in
voids by hydrodynamical cosmological simulations. Due to the cosmic
ultra-violet background radiation low-mass haloes show generally are reduced
baryon fraction. We determine the characteristic mass below which dwarf
galaxies are baryon deficient. We show that the circular velocity below which
the accretion of baryons is suppressed is approximately 40 km/s. The suppressed
baryon accretion is caused by the photo-heating due to the UV-background. We
set up a spherical halo model and show that the effective equation of state of
the gas in the periphery of dwarf galaxies determines the characteristic mass.
This implies that any process which heats the gas around dwarf galaxies
increases the characteristic mass and thus reduces the number of observable
dwarf galaxies.
