Dark energy is a mysterious form of energy that is thought to make up about 68% of the total energy content in the universe. It is believed to be responsible for the observed accelerated expansion of the universe. The existence of dark energy was first proposed in the late 1990s based on observations of distant supernovae, and has since become a major focus of research in cosmology. Scientists are still working to understand the nature of dark energy and its implications for the future of the universe.