While dark energy sounds like that aspect of the Force used by Darth Vader and his Emperor, and who knows, perhaps it could be - it probably is not. Dark energy is primarily an explanation rather than a determined reality. It is the name given to a presumed universal energy to explain the difference between human theories in astronomy and physics and the observable facts made available through improved technology such as the Hubble Space Telescope. While some theoretical physicists may consider the concept of dark energy to be a theory, it is more legitimate in scientific terms to call it an hypothesis, and to some, even that is pushing legitimacy.
An hypothesis is basically an educated guess with the potential to be tested for accuracy and legitimacy. Supposedly, to be an hypothesis rather than just a guess, there needs to be a conceivable way of testing it. If such testing produces results that can be repeated by some other tester and do not refute the 'guess' in any way, shape or form, are sufficiently extensive, and then pass a peer review by acknowledged scientists in that specific field, the hypothesis may be elevated to the status of theory.
In previous centuries theories were called laws by scientists, such as Newton's Three Laws of Motion. Today it is accepted that theories are simply the best we can determine at this point in time, that we know enough to realize how little we know and therefore calling them Laws would be hubris, excessive pride.
The concept of dark energy is the result of our astronomical observations not meeting and confirming our preconceived ideas and beliefs. Once religious dogma was finally removed from the picture and the Earth was no longer considered the center of the universe, the predominant theory was that the universe existed in a steady state. As our technological capabilities improved, enabling more detailed and extensive observation of distant stars, galaxies and nebulae, the concept of the 'Big Bang' became and continues to be the primary hypothesis for the establishment of the universe. Astronomical observations show that the universe is expanding, so the conclusion is that it must have started from a point source.
The Big Bang theory presumes an initial massive explosion of highly exotic short term particles that created the universe some 13.7 billion years ago. The force of that explosion initiated an expansion of energy and matter resulting in the billions of galaxies we can observe in our skies. Galaxies that are observably moving further apart.
However, that presupposes an initial impetus from that originating Big Bang. The galaxies therefore should be expanding at a steady velocity. In 1998, a study of type 1a supernovae showed that the expansion of the universe is not remaining constant, nor is it slowing down as might possibly occur due to friction from interacting with interstellar dust. It is speeding up!
No-one knows why, and no reputable scientist is willing to offer a guess. All we currently know about physics says that for acceleration to occur, force (energy) needs to be applied. As the expansion of the universe is observably accelerating we have to believe on the basis of our current knowledge that some form of energy is causing it to do so, even though we cannot detect or measure that energy. Because we can't, we are guessing it exists, and since we can't see it, we are quite reasonably naming it “Dark Energy”. Nevertheless, at this time we neither know if dark energy truly exists nor, if it does, what it actually is.