Dark Energy is supposed to substantiate the theory of ; the substance itself is a hypothetical one. There is also a little controversy, though: Albert Einstein provided the idea with his but without evidence it fell by the wayside - the result being its exclusion from certain fields of physics, while still being useful to others, bringing about a division (for lack of a better word.)
From memory, I think Einstein's 'discovery' was the consequence of trying to fix something that wasn't broken, to make his theory account for a static universe - when he witnessed evidence of he rescinded (I recall he noted later, something along the lines of his reasoning for the constant being the biggest mistake of his life, but I'll need to look up a reference to quote anything.)
According to the Wikipedia link I provided it seems there has since been evidence for this dark energy since. Differing definitions keep cropping up, and few will still be being considered as candidates but the problem still goes unsolved, for the most part. There is also another form, and hence doubtlessly a discovery story, but of which I know nothing.
When was dark energy discovered How has that discovery changed our view of the universe
The first suggestion for dark energy from observed data happened in 1992, when Gyorgy Paal and his collaborators published a paper on the subject. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding at an accelerating rate. Thanks for using