  Over the past decade, a consensus picture has emerged in which roughly a
quarter of the universe consists of dark matter. The observational evidence for
the existence of dark matter is reviewed: rotation curves of galaxies, weak
lensing measurements, hot gas in clusters, primordial nucleosynthesis and
microwave background experiments. In addition, a new line of research on Dark
Stars is presented, which suggests that the first stars to exist in the
universe were powered by dark matter heating rather than by fusion: the
observational possibilities of discovering dark matter in this way are
discussed.
