  Molecular gas has been searched for and found in unexpectedly large
quantities in some collisional debris of interacting galaxies: HI-rich tidal
tails, bridges and collisional rings. It was so far observed through millimeter
observations of the CO line and detected towards or near regions of
star-formation associated to dense condensations of the atomic hydrogen. The
discovery of cool H2 at distances greater than 50 kpc from the parent
(colliding) galaxies, whereas the external disk of spirals is generally
considered to be CO-poor, raised question on its origin and favored the
hypothesis of a local production out of collapsed HI clouds. However recent
observations of a diffuse CO component along tidal debris have challenged this
idea. Another recent puzzle is the measurement in the collisional debris of two
interacting systems and four recycled objects of a missing mass, whereas no
dark matter is expected there. One debated interpretation is that this unseen
component is cold, "invisible" molecular gas initially present in the disk of
spirals.
