What is dark energy?

Question

Is this something that is entirely made up to satisfy some conservation law, or is there any real evidence for dark energy?

Answer 1

Scientists based on experimenets so for, have come to the concllusion that it exists, However certainty on; the subject is only when further results are obtained from the scientists as can be seen from the following:
Dark energy is hardly science fiction, although no less intriguing and full of mystery for being real science. Various types of dark energy have been proposed, including a cosmic field associated with inflation; a different, low-energy field dubbed "quintessence"; and the cosmological constant, or vacuum energy of empty space.
"The universe is made mostly of dark matter and dark energy," says Saul Perlmutter, leader of the Supernova Cosmology Project headquartered at Berkeley Lab, "and we don't know what either of them is." He credits University of Chicago cosmologist Michael Turner with coining the phrase "dark energy" in an article they wrote together with Martin White of the University of Illinois for Physical Review Letters.
In an article titled "The Cosmic Triangle: Revealing the State of the Universe," which appears in the May 28, 1999 issue of the journal Science, a group of cosmologists and physicists from Princeton University and Lawrence Berkeley National Laboratory survey the wide range of evidence which, they write, "is forcing us to consider the possibility that some cosmic dark energy exists that opposes the self-attraction of matter and causes the expansion of the universe to accelerate."
The Cosmic Triangle is the authors' way of presenting the major questions cosmology must answer: "How much matter is in the universe? Is the expansion rate slowing down or speeding up? And, is the universe flat?"
The possible answers are values for three terms in an equation that describes the evolution of the universe according to the general theory of relativity. By plotting the best experimental observations and estimates within the triangle, scientists can make preliminary choices among competing models.
The mass density of the universe is estimated by deriving the ratio of visible light to mass in large systems such as clusters of galaxies, and in several other ways. For several decades the evidence has been building that mass density is low and that most of the matter in the universe is dark.
VR

Answer 2

Dark energy has been proposed from scientific measurements of the speed galaxies and galaxy clusters are moving away from each other. So it is a theory based on evidence from observation by astronomers.

The amount galaxis are moving away from each other is faster than can be explained by the big bang and gravity. So they propose that there must be a dark energy which has repulsive properties like anti-gravity. No one can ever observed the anti-energy itself, just the effect of it, so that is why it is called "dark" energy.

Answer 3

In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and has strong negative pressure.