In this chapter we review dark matter, whose physical nature remains unknown 90 years after astronomer Fritz Zwicky found the first evidence for it. We go over the historical evidence for dark matter, than review modern indications for the existence of this component. We then review particle candidates for dark matter, outlining the properties that dark matter of each particle candidate would have. Next we go over the direct, indirect, and laboratory searches for dark matter. We end by discussing a dramatically different alternative (to particles) for the nature of dark matter -- that of a modified theory of gravity (MOND).

Scientists and philosophers of science are most impressed by theories that make successful, novel predictions: that predict surprising facts in advance of their experimental or observational confirmation. There is a theory of cosmology that has repeatedly been successful in this privileged way, but it is not the standard, or 𝚲CDM, model. It is Mordehai Milgrom’s MOND theory (MOdified Newtonian Dynamics). Unlike the standard model, MOND does not postulate the existence of dark matter. Observations that are explained in the standard model by invoking dark matter are explained in MOND by postulating a change in the laws of gravity and motion.

In 1983, the physicist Mordehai Milgrom initiated a new research program in cosmology, called MOND (for MOdified Newtonian Dynamics), or Milgromian dynamics. In three papers, Milgrom proposed a set of postulates describing how Newton’s laws of gravity and motion should be changed in regimes of very low acceleration. Milgrom’s postulates were designed to explain the asymptotic flatness of galaxy rotation curves, without the necessity of postulating the existence of “dark matter”. Milgrom showed that a number of other, novel predictions follow from his three postulates, and proposed these predictions as tests of the theory. Milgrom also proposed a set of guiding principles for how his nascent theory should be developed toward a more complete theory of gravity and cosmology.