Dark Matter
  Observations in the optical, in X-rays, and gravitational lensing of galaxies, clusters of galaxies, and large-scale structure are beginning to provide clues to the dark matter problem. I review the impact of these observations on some of the main questions relating to dark matter: How much dark matter is there? Where is it located? What is the nature of the dark matter? and what is the amount of baryonic dark matter. 
Neta Bahcall (Princeton University Observatory)