  We study statistical properties of voids in the distribution of mass,
dark-matter haloes and galaxies (B_J<-16) in a LCDM numerical simulation
populated with galaxies using a semi-analytic galaxy formation model(GALFORM,
Cole et al. 2000). We find that the properties of voids selected from GALFORM
galaxies are compatible with those of voids identified from a population of
haloes with mass M>10^11.5 M_sun/h, similar to the median halo mass,
M_med=10^11.3 M_sun/h. We also find that the number density of galaxy- and
halo-defined voids can be up to two orders of magnitude higher than
mass-defined voids for large void radii. As expected, there are outflow
velocities which show their maximum at larger void-centric distances for larger
voids (well described by a linear relation). The void-centric distance where
this maximum occurs, follows a suitable power law fit of the form,
log(d_vmax)=(r_void/A)^B. At sufficiently large distances, we find mild infall
motions onto the sub-dense regions. We find that a similar analysis in redshift
space would make both outflows and infalls to appear with a lower amplitude. We
also find that the velocity dispersion of galaxies and haloes is larger in the
direction parallel to the void walls by ~10-20%. We analyse the void-tracer
cross-correlation functions in real- and redshift-space as a function of
separation, and also as a function of separations parallel and perpendicular to
the line of sight. The distortion pattern observed in xi(sigma,pi) is that of
an elongation along the line of sight which extends out to large separations.
Positive xi contours evidence finger-of-god motions at the void walls.
Elongations along the line of sight are roughly comparable between galaxy-,
halo- and mass-defined voids. (Abridged)
