In this lecture Prof Susskind reviews the basic equations of cosmology, and discusses the meaning of these and the implied structure of the universe. We further discuss dark matter. Prof Susskind introduces the fact that most of the matter in the universe is dark. Prof Susskind derives the RFW from energy conversation, assuming it was homogeneous and isotropic. He further derives the fact that the universe doesn’t grow linearly, but rather on a 2/3rd’s power of time. This is the matter dominated universe expansion. Prof Susskind discusses that the universe from the age of about 100K years to about 2Billion years of age, followed the matter dominated law of expansion. Prof Susskind introduces the fact that before matter dominated expansion the universe was dominated by radiation dominated expansion, mostly photon based. Discuss that the universe is largely made up of photons. That photons don’t have mass, but they do have energy (rest energy). Prof Susskind discusses why the universe was initially radiation dominated. He further discusses that likely the dark matter may be originated in weakly interacting particles we just haven’t discovered yet. Prof Susskind discusses why dark matter is no longer considered to be hydrogen based.