Energy of currently unknown origin that has become the dominant influence on cosmic expansion and is currently causing that expansion to accelerate. Dark energy is thought to account for about 70% of all the energy in the Universe. It is not thought to be associated with dark matter. It is related to Einstein’s cosmological constant: this constant can be thought of as an in-built property of space and time to expand, or as a mysterious substance within space and time that causes expansion. If taking this latter view, the cosmological constant can be thought of as a particular (constant) type of dark energy.