The history of dark energy can be traced back to the early 20th century, when Albert Einstein introduced the cosmological constant term in his theory of general relativity, in order to balance the attractive force of gravity and account for a static universe. However, after Edwin Hubble discovered that the universe is expanding, Einstein abandoned the cosmological constant, calling it his “greatest blunder”.
In the late 20th century, several observations, such as the cosmic microwave background radiation, the large-scale structure of the universe, and the type Ia supernovae, suggested that the universe is not only expanding, but also accelerating, implying the existence of a repulsive force that counteracts gravity on large scales.