From what has been described so far, it should be clear that the structure and evolution of spiral galaxies depend largely on the amount and distribution of dark matter actually present in these systems. (This statement would apply in even stronger terms to evolutionary scenarios in which galaxy-galaxy interactions play a major role.) In modeling of the basic state (e.g., see Sections 14.4 and 14.5) and discussion of several dynamical mechanisms (e.g., see Section 18.2 or Chapter 19), we have recognized that, in general, a dark halo may be present. However, we have left open the possibility that in some galaxies the halo may dominate, whereas in others it may be relatively small or even insignificant from the dynamical point of view. In this chapter we briefly review the arguments that have brought us to believe that dark matter is indeed present in spiral galaxies. This will also show that many issues remain far from being fully understood.

Starting with the studies by Kapteyn and Oort of the stellar motions in the solar neighborhood (see Section 14.1.2) and the studies by Zwicky and Smith of the galaxy motions inside clusters, it has long been suspected that the Universe contains much more matter than meets the eye, that is, through the telescopes at our disposal.