  We examine the baryon content of low-mass {\Lambda}CDM halos
$(10^8<M_{200}/{\rm M_\odot}<5\times 10^{9})$ using the APOSTLE cosmological
hydrodynamical simulations. Most of these systems are free of stars and have a
gaseous content set by the combined effects of cosmic reionization, which
imposes a mass-dependent upper limit, and of ram pressure stripping, which
reduces it further in high-density regions. Halos mainly affected by
reionization (RELHICs; REionization-Limited HI Clouds) inhabit preferentially
low-density regions and make up a population where the gas is in hydrostatic
equilibrium with the dark matter potential and in thermal equilibrium with the
ionizing UV background. Their thermodynamic properties are well specified, and
their gas density and temperature profiles may be predicted in detail. Gas in
RELHICs is nearly fully ionized but with neutral cores that span a large range
of HI masses and column densities and have negligible non-thermal broadening.
We present predictions for their characteristic sizes and central column
densities: the massive tail of the distribution should be within reach of
future blind HI surveys. Local Group RELHICs (LGRs) have some properties
consistent with observed Ultra Compact High Velocity Clouds (UCHVCs) but the
sheer number of the latter suggests that most UCHVCs are not RELHICs. Our
results suggest that LGRs (i) should typically be beyond 500 kpc from the Milky
Way or M31; (ii) have positive Galactocentric radial velocities; (iii) HI sizes
not exceeding 1 kpc, and (iv) should be nearly round. The detection and
characterization of RELHICs would offer a unique probe of the small-scale
clustering of cold dark matter.
