  We provide new post-COSTAR data on one sightline (Mrk 421) and updated data
from another (I Zw 1) from our Hubble Space Telescope (HST) survey of
intergalactic Lyman-alpha clouds located along sightlines to four bright
quasars passing through well-mapped galaxy voids (16,000 km/s pathlength) and
superclusters (18,000 km/s). We report two more definite detections of low-z
Lyman-alpha clouds in voids: one at 3047 km/s (heliocentric) toward Mrk 421 and
a second just beyond the Local Supercluster at 2861 km/s toward I Zw 1,
confirming our earlier discovery of Lyman-alpha absorption clouds in voids
(Stocke et al. 1995). We have now identified 10 definite and 1 probable
low-redshift neutral hydrogen absorption clouds toward four targets, a
frequency of approximately one absorber every 3400 km/s above 10^12.7 cm^-2
column density. Of these 10 absorption systems, 3 lie within voids; the
probable absorber also lies in a void. Thus, the tendency of Lyman-alpha
absorbers to ``avoid the voids'' is not as clear as we found previously. If the
Lyman-alpha clouds are approximated as homogeneous spheres of 100 kpc radius,
their masses are ~ 10^9 Msun (about 0.01 times that of bright L* galaxies) and
they are 40 times more numerous, comparable to the density of dwarf galaxies
and of low-mass halos in numerical CDM simulations. The Lyman-alpha clouds
contribute a fraction Omega_cl ~ 0.003 h75^-1 to the closure density of the
universe, comparable to that of luminous matter. These clouds probably require
a substantial amount of non-baryonic dark matter for gravitational binding.
They may represent extended haloes of low- mass protogalaxies which have not
experienced significant star formation or low-mass dwarf galaxies whose star
formation ceased long ago, but blew out significant gaseous material.
