Abstract
For the first time in human thought it is now
possible to observationally determine how much matter is in the Universe as a
whole. These observations strongly support the “Concordance Model” of Hot Big
Bang Cosmology, and reinforce earlier indications that ordinary matter (atoms,
nuclei and electrons) make up at present at most 4% of the total of the
Universal energy density. The big surprise was that the rest consists of *two*
kinds of unknown forms of matter: the so-called Dark Matter and Dark Energy.
This talk summarizes for non-specialists the various lines of evidence for
their existence, and some of the theoretical ideas which have been proposed to
account for their properties
