The story begins in the 1930s with a Caltech astronomy professor. Fritz Zwicky, while analyzing his observations of the Coma Cluster in 1933, became the œrst to apply the virial theorem to infer the existence of unseen matter in this cluster. This unseen matter is now called dark matter (Zwicky, 1933; see also Zwicky, 1937). Using the virial theorem, he was able to infer the average mass of galaxies within the cluster, obtained a value 500 times greater than expected from their optical luminosity, and proposed that most of the matter was “dark (cold) matter” (DM; Rubin, 2001). Much more work of this type has been done since Zwicky, including the discovery by x-ray space telescopes of large amounts of hot gas in clusters through their x-ray emissions (not seen by Zwicky), and the result is that the missing mass, i.e., dark matter in galaxy clusters, is about 2.5-5 times the total directly observed luminosity mass (clearly baryonic mass). Thus, when these observations are extrapolated to the total mass of the Universe, this gives a fraction of mass for a ¬at universe ΩM ~ 10%–30% (Reiprich and Böhringer, 2002). On the other hand, Big Bang nucleosynthesis (BBNS) sets limits on the baryonic matter fractions at Ωb ~ 4%–5% (Spergel et al., 2007; for an entertaining overview of why we need dark matter, see Siegel, 2008).