  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.
