Abstract
At the end of the 20th century, the accelerating expansion of the universe was discovered. The cosmological term, which was introduced by Einstein and abandoned later by himself, attracts much attention recently for that reason. The cosmological term is unnatural in a theoretical viewpoint, and the idea of dark energy, which is a generalization of the cosmological term, was introduced and investigated. At the moment, theoretical understanding of the dark energy is fairly poor, and much observational data are required. In this article, recent research for the dark energy is reviewed.
