  Recent cosmological observations suggest that nearly seventy per cent of the
energy density in the universe is unclustered and has negative pressure.
Several conceptual issues related to the modeling of this component (`dark
energy'), which is driving an accelerated expansion of the universe, are
discussed with special emphasis on the cosmological constant as the possible
choice for the dark energy. Some curious geometrical features of a universe
with a cosmological constant are described and a few attempts to understand the
nature of the cosmological constant are reviewed.
