Add to ORKGLooking up, at night, one can immediately notice that the universe is almost empty. On a bright night, at first glance, a few nearby bright stars are visible. It is only after ones eyes are accustomed to the dark that the milky way becomes apparent as a hazy whitish band across the sky. External galaxies are equally faint and considerable effort is needed to study such objects. Kinematical studies of galaxies are most often carried out by gathering information of emission lines of the gaseous component. Most of the emisssion is then concentrated in a few discrete and bright lines for instance Ha or the 21 cm neutral hydrogen line. The gas is a good tracer of the potential but constitutes only a minor fraction of the mass content of a galaxy...