One of the biggest challenges in present day cosmology is to explain the "missing mass" which forms over 95% of the content of the universe. This emerged as important in the 1970's when astronomers compared systematically the gravitational masses of galaxies and galaxy groups with their total content of stars gas and dust, finding that over nine tenths of the gravitational mass could not be accounted for. It was also realised that the discs of spiral galaxies would be unstable without the presence of a massive dark halo.
Then the inflationary model which explains the observed isotropy of the cosmic microwave background (CMB) radiation, implies a "flat" universe while the light element abundances predicted in the standard model Big Bang cosmology show measured values implying that "normal" baryonic matter can make up less than 5% of the mass needed for the universe to be flat. Our best models accounting for the observed large-scale distribution of mass and velocity of galaxies and clusters start with density fluctuations in an inflationary universe, and require a dominant mass component of non-baryonic matter non-relativistic at the time of its decoupling from the background radiation. The best of these "cold dark matter" (CDM) models, however, reque a CDM density only -30% of that required closing the universe, in apparent contradiction with inflationary predictions.