title: Introduction.
abstract: It is embarrassing that 95% of the Universe is unaccounted for. Galaxies and larger-scale cosmic structures are composed mainly of "dark matter", whose nature is still unknown. Favoured candidates are weakly interacting particles that have survived from the very early Universe, but more exotic options cannot be excluded. (There are strong arguments that the dark matter is not composed of baryons.) Intensive experimental searches are being made for the "dark" particles (which pervade our entire Galaxy), but we have indirect clues to their nature too. Inferences from galactic dynamics and gravitational lensing allow astronomers to "map" the dark-matter distribution; comparison with numerical simulations of Galaxy formation can constrain (for example) the particle velocities and collision cross-sections; and, of course, progress in understanding the extreme physics of the ultra-early Universe could offer clues to what particles might have existed then, and how many would have survived. The mean cosmic density of dark matter (plus baryons) is now pinned down to be only ca.30% of the so-called critical density corresponding to a "flat" Universe. However, other recent evidence-microwave background anisotropies, complemented by data on distant supernovae-reveals that our Universe actually is "flat", but that its dominant ingredient (ca.70% of the total mass energy) is something quite unexpected: "dark energy" pervading all space, with negative pressure. We now confront two mysteries. (i) Why does the Universe have three quite distinct basic ingredients-baryons, dark matter and dark energy-in the proportions (roughly) 5%, 25% and 70%? (ii) What are the (almost certainly profound) implications of the "dark energy" for fundamental physics?
authors.affiliation: Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Rees