Title: "Dark energy in the Universe"

Recent numerous observational data obtained from such independent sources as angular anisotropies of the cosmic microwave background radiation, large-scale gravitational clustering of galaxies and their clusters and observations of supernovae explosions at high redshifts prove convincingly that about 70% of the total energy density of matter in the present Universe is due to a new kind of matter in the Universe ("dark energy") which is non-baryonic, has negative pressure which modulus is very close to dark energy density (if the Einsteinian form of gravity field equations is assumed) and remains unclustered at all scales where clustering of baryons and dust-like cold dark matter is seen. I discuss different forms of phenomenological description of dark energy properties, consider a possibility of decaying dark energy and present limits on this process from supernovae data, and make a brief review of different theoretical models of dark energy including those in which it has a purely geometrical origin. However, the simplest possibility of dark energy being a cosmological constant and nothing more still remains the best fit to all existing observational data.