  High speed collisions, although current in clusters of galaxies, have long
been neglected, as they are believed to cause little damages to galaxies,
except when they are repeated, a process called harassment. In fact, they are
able to produce faint but extended gaseous tails. Such low-mass, starless,
tidal debris may become detached and appear as free floating clouds in the very
deep HI surveys that are currently being carried out. We show in this paper
that these debris possess the same apparent properties as the so-called "Dark
Galaxies", objects originally detected in HI, with no optical counterpart, and
presumably dark matter dominated. We present a numerical model of the prototype
of such Dark Galaxies - VirgoHI21 -, that is able to reproduce its main
characteristics: the one-sided tail linking it to the spiral galaxy NGC 4254,
the absence of stars, and above all the reversal of the velocity gradient along
the tail originally attributed to rotation motions caused by a massive dark
matter halo and which we find to be consistent with simple streaming motions
plus projection effects. According to our numerical simulations, this tidal
debris was expelled 750 Myr ago during a fly-by at 1100 km/s of NGC 4254 by a
massive companion which should now lie at a projected distance of about 400
kpc. A candidate for the intruder is discussed. The existence of galaxies that
have never been able to form stars had already been challenged based on
theoretical and observational grounds. Tidal collisions, in particular those
occurring at high speed, provide a much more simple explanation for the origin
of such putative Dark Galaxies.
