Dark Energy is a hypothetical form of energy that exerts a negative, repulsive pressure, behaving like the opposite of gravity. It is thought to permeate all of space and is responsible for the observed accelerated expansion of the universe. Current models suggest it constitutes approximately 68% of the total energy-mass of the universe.
The evidence for dark energy was discovered in 1998 by two independent teams of astronomers studying distant Type Ia supernovae. They found that the universe's expansion is speeding up, not slowing down as previously thought. The term 'dark energy' was coined by cosmologist Michael Turner.
Understanding dark energy is one of the biggest challenges in modern physics and cosmology. Major projects like the Dark Energy Survey and the Euclid space telescope are dedicated to mapping the universe to constrain the properties of this mysterious force.