What is a galaxy? Stars, gas, some dust and surrounding them an invisible dark matter halo. The discovery a few years ago of VirgoHI21, a so-called dark galaxy without stars, was met with great interest by researchers, who were troubled by the existence of a specimen which was not predicted by theoretical models. A French team from the laboratory AIM(1) (CEA, University Paris Diderot, CNRS) has just shown with the help of digital simulations that VirgoHI21 is in fact probably just collisional debris from a past encounter between two massive galaxies. The massive dark galaxy prototype is thus disproved. This research will be published in Astrophysical Journal in February 2008.