Dark energy is believed to be the component of the Universe whose repulsive gravity is presently accelerating its expansion. It was first detected by two teams of astrophysicists in 1998. Although its nature is not well understood, it comprises the bulk (about 70%) of the mass-energy of the Universe. Albert Einstein originally suggested a form of Dark Energy might exist (which he called the cosmological constant) about 80 years ago. Today, understanding the nature of Dark Energy is one of the biggest challenges in physics.