Dark Energy: Mystery of the Millennium

Nearly seventy per cent of the energy density in the universe is unclustered and exerts negative pressure. This conclusion -- now supported by numerous observations -- poses the greatest challenge for theoretical physics today. I discuss this issue with special emphasis on the cosmological constant as the possible choice for the dark energy. Several curious features of a universe with a cosmological constant are described and some possible approaches to understand the nature of the cosmological constant are reviewed. In particular, I show how some of the recent ideas, related to a thermodynamic route to gravity, allow us to: (i) create a paradigm in which the bulk value of cosmological constant is irrelevant and (ii) obtain the correct, observed, value for the cosmological constant from vacuum fluctuations in a region confined by the deSitter horizon.