The concept of Dark Energy gives explanation to the observed accelerating expansion of the universe, which otherwise would be decelerating. Dark Energy can be inserted into General Relativity (GR) in the manner of the Cosmological Constant (Λ) Einstein originally included to maintain the universe's size. With Dark Energy, such a term is theorized to be sufficiently large to increase the size of the universe, and it isn't clear that it is constant.
The acceleration of the universe's expansion is evidenced by the geometry of viewable space, both from the frequency of supernovae (SN) and in the Cosmic Microwave Background (CMB). The Supernova evidence dates from the late 1990s and the CMB evidence has become apparent since then. The concept is very well accepted.
Alternatives to the Dark Energy hypothesis would be other adjustments to General Relativity (e.g., DGP Gravity), or that it is an illusion, e.g., its evidence stemming from selection bias.
Cosmological Constant (Λ)
Dark Energy Survey (DES)
Dark Energy Spectroscopic Instrument (DESI)
General Relativity (GR)