  Using data from the ALFALFA Arecibo HI survey of galaxies and the Virgo
cluster X-ray pressure profiles from XMM-Newton, we investigate the possibility
that starless dark HI clumps, also known as "dark galaxies", are supported by
external pressure in the surrounding intercluster medium. We find that the
starless HI clump masses, velocity dispersions and positions allow these clumps
to be in pressure equilibrium with the X-ray gas near the virial radius of the
Virgo cluster. We predict the sizes of these clumps to range from 1-10 kpc, in
agreement with the range of sizes found for spatially resolved HI starless
clumps outside of Virgo. Based on the predicted HI surface density of the Virgo
sources, as well as a sample of other similar resolved ALFALFA HI dark clumps
with follow up optical/radio observations, we predict that most of the HI dark
clumps are on the cusp of forming stars. These HI sources therefore mark the
transition between starless HI clouds and dwarf galaxies with stars.
