What actually Dark energy is ? [duplicate]

I recently read an article on dark matter and I could understand this things given as follows All the ordinary matter we can find accounts for only about 4 percent of the universe. We know this by calculating how much mass would be needed to hold galaxies together and cause them to move about the way they do when they gather in large clusters. Another way to weigh the unseen matter is to look at how gravity bends the light from distant objects. Every measure tells astronomers that most of the universe is invisible.

I want to say that the universe must be full of dark clouds of dust or dead stars and be done with it, but there are persuasive arguments that this is not the case. First, although there are ways to spot even the darkest forms of matter, almost every attempt to find missing clouds and stars has failed. Second, and more convincing, cosmologists can make very precise calculations of the nuclear reactions that occurred right after the Big Bang and compare the expected results with the actual composition of the universe. Those calculations show that the total amount of ordinary matter, composed of familiar protons and neutrons, is much less than the total mass of the universe. Whatever the rest is, it isn't like the stuff of which we're made. But what actually is dark energy ? How should I connect dark energy with dark matter ?

• possible duplicate of Dark matter and dark energy – Kyle Kanos Jul 18 '14 at 14:49
• @KyleKanos You are closing this as a duplicate of a question that was closed as a duplicate of a different question? Wouldn't it make more sense to say it's a duplicate of the non-closed question? If it isn't a duplicate of that, then either this question cannot be a duplicate of the one you linked or the one you linked cannot be a duplicate of the one it links to. One cannot say A=B and B=C and $A\ne C$. – Jim Jul 18 '14 at 15:10
• @Jim: Personally I think $B\neq C$ as $B$ is about DM and DE while $C$ is about DM only (so, IMO, it's not a duplicate). $A$ is a duplicate of $B$ because it is about both DM & DE. – Kyle Kanos Jul 18 '14 at 15:12
• @KyleKanos so then should we not vote to reopen B? – Jim Jul 18 '14 at 15:14
• @Jim: although the proposed duplicate is itself closed as a duplicate it has a couple of good answers. – John Rennie Jul 18 '14 at 15:15