Mass discrepancies in the Universe: MACHOs, WIMPs, ALPs, MOND and all that

A variety of astronomical as well as cosmological observations starting from the early 1930s indicate that there exist large discrepancies between the dynamical estimates of the amount of gravitating mass in the Universe and the amount of visible mass that we can "see" and measure. The dynamical mass estimates are based on usual Newton/Einstein-ian dynamics. The mass discrepancy (MD) exists essentially on all scales ranging from (sub)galactic scales all the way up to the largest scale structures seen in the Universe today. The most popular resolution of the MD problem hinges on the hypothesis of existence of a new form of matter that emits no electromagnetic radiation at any wavelength thus making it invisible. The physical nature of the constituent(s) of this hypothetical "Dark Matter" (DM), that dominates the total amount of gravitating mass in the Universe, however, remains unknown till date. There exist a large number of ideas and suggestions as to the possible composition of the DM --- Massive Astrophysical Compact Halo Objects (MACHOs), Weakly Interacting Massive Particles (WIMPs), Axion-Like Particles (ALPs), and so on. In addition, alternative explanation of the MD problem in terms of the so-called MOdified Newtonian Dynamics (MOND) (without the need for any DM) has also been suggested. In this talk we review the MD problem, the DM hypothesis and various suggested candidates of DM, and the basic ideas behind worldwide efforts towards detection of some of the suggested candidates of DM in laboratory experiments. Finally, we shall briefly discuss the basic ideas behind and issues with the MOND hypothesis of resolution of the MD problem.