Dark Energy is a hypothetical form of energy that permeates all of space and is believed to be responsible for the observed acceleration in the expansion of the universe. It was first proposed as an explanation after astronomers discovered, through observations of distant supernovae, that the universe's expansion was not slowing down but instead accelerating.
The exact nature of dark energy remains unknown, although it has been suggested to be a property of space itself or a new type of fundamental field. Some theories propose that dark energy could make up as much as 70% of the total mass-energy content of the universe. Its effects are most pronounced on cosmological scales and its precise properties remain one of the biggest mysteries in modern physics.