The concept of dark energy gives explanation to the observed accelerating expansion of the universe, which otherwise would be decelerating due to gravity. Dark energy can be inserted into general relativity's field equation in the manner of the cosmological constant that Einstein originally included to maintain the universe's size. With dark energy, such a term is theorized to be sufficiently large that the size of the universe increases, and it is presumed unlikely to be constant over time.
The acceleration of the universe's expansion is evidenced by the geometry of viewable space, both from the frequency of supernovae and in the cosmic microwave background. The supernova evidence dates from the late 1990s and since then, the CMB's indication of a flat universe has been interpreted as evidence. The concept is very well accepted.
Alternatives to the dark energy hypothesis include adjustments to general relativity (e.g., DGP gravity), or that it is an illusion, such as might result from some kind of selection bias in the gathering of the evidence.
The observed acceleration could be accomplished by some kind of vacuum energy, i.e., present even in a vacuum. An obvious candidate is the zero-point energy predicted by quantum theory but the measured acceleration of the universe's expansion requires a vacuum energy sixty orders-of-magnitude (or more) smaller than this theoretical zero-point energy, a situation which has been cited as a strikingly large gap between theory and measurement.