Dark energy is a theoretical form of energy that is believed to permeate all of space and contribute to the observed acceleration of the expansion of the universe. It is called "dark" because it does not emit, absorb, or interact with electromagnetic radiation (such as light) that is detectable by our current instruments. However, its effects can be inferred through its impact on the large-scale structure and dynamics of the universe.
Unlike dark matter, which is also invisible to our direct observation, dark energy is not associated with the gravitational effects on galaxies and galaxy clusters. Dark matter interacts through gravity and exerts a gravitational pull on visible matter, influencing the formation and movement of structures in the universe. Dark energy, on the other hand, is thought to be responsible for the observed accelerated expansion of the universe.
The nature of dark energy is still not well understood, and it remains one of the biggest mysteries in modern physics and cosmology. It is often associated with the concept of vacuum energy or the cosmological constant, proposed by Albert Einstein. The exact origin and composition of dark energy are subjects of ongoing research and investigation.