Dark energy is theoretical to fit the rotational speed of galaxies, the gravitational bending of light around galaxies and galaxy clusters and the estimated expansion rate of the universe. I don't know if it is something real or something we don't understand yet.
Dark energy was proposed to replace Einstein’s “fudge factor”, the cosmological constant, to account for the universe expanding more quickly than it should according to the known forces of the universe. I guess “energy we can’t detect” is more palatable to scientists than just introducing another force.
Dark matter is what is needed to correct for nature (not just the speed) of galactic rotation. It’s an even worse “fudge factor”, since it is basically able to magically appear wherever scientists need it and never where they don’t.