2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/102602Observations suggest that nearly seventy per cent of the energy density in the universe is unclustered and exerts negative pressure. Theoretical understanding of this component (`dark energy'), which is driving an accelerated expansion of the universe, is {\it the} problem in cosmology 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 discussed.Invited Review Talk at the 29th International Cosmic Ray Conference, Aug 3-10, 2005, Pune, India; to appear in the proceedings; 15 pages; 2 figuresAstrophysicsGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theorytext