  Debris sent into the intergalactic medium during tidal collisions can tell us
about several fundamental properties of galaxies, in particular their missing
mass, both in the form of cosmological Dark Matter and so-called Lost Baryons.
High velocity encounters, which are common in clusters of galaxies, are able to
produce faint tidal debris that may appear as star-less, free floating HI
clouds. These may be mistaken for Dark Galaxies, a putative class of gaseous,
dark matter dominated, objects which for some reason never managed to form
stars. VirgoHI21 is by far the most spectacular and most discussed Dark Galaxy
candidate so far detected in HI surveys. We show here that it is most likely
made out of material expelled 750 Myr ago from the nearby spiral galaxy NGC
4254 during its fly--by at about 1000 km/s by a massive intruder. Our numerical
model of the collision is able to reproduce the main characteristics of the
system: in particular the absence of stars, and its prominent velocity
gradient. Originally attributed to the gas being in rotation within a massive
dark matter halo, we find it instead to be consistent with a combination of
simple streaming motion plus projection effects (Duc & Bournaud, 2007). We
discuss several ways to identify a tidal origin in a Dark Galaxy candidate and
illustrate the method using another HI system in Virgo, VCC 2062, which is most
likely a Tidal Dwarf Galaxy (Duc et al., 2007). Now, whereas tidal debris
should not contain any dark matter from the halo of their parent galaxies, it
may exhibit missing mass in the form of dark baryons, unaccounted for by
classical observations, as recently found in the collisional ring of NGC 5291
(Bournaud et al., 2007) and probably in the TDG VCC 2062. These "Lost Baryons"
must originally have been located in the disks of their parent galaxies.
