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I've heard a lot about Dark Energy, and how it's supposed to be one of the most powerful forces in the universe. What is it, and how was it discovered?

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Dark Energy was discovered in 1998 when two separate teams of astronomers (see here and here) studied Type Ia supernovae. The use of Type Ia supernovae is particularly important because they have a very specific luminosity as a result of how they occur. Both groups independently found that the distances to the galaxies which hosted the supernovae they were observing were farther than had been found using other methods and than would be consistent with a universe expanding at constant velocity.

Dark Energy is the name given to the acceleration of the expansion of the universe. While it is a powerful force on the scale of the entire universe, it should be noted that it is not a very powerful force on scales smaller than the distances between galaxies. In fact, even the gravitational force between the Milky Way and Andromeda appears to be stronger than the 'push' from dark energy at that scale.

There have been several theoretical constructs that have attempted to explain Dark Energy, but none of them adequately describe its effects. In fact, most are incorrect by many orders of magnitude. For a review of some of these constructs, see this paper.

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In addition to SNe Ia, observations of large scale structure, galaxy clusters, and the cosmic microwave background all strongly suggest the presence of dark energy. –  Jeremy Jun 2 '11 at 17:58
    
@Jeremy: True. I have focused my answer on the most well known pair of experiments that led to the discovery of Dark Energy, but many other observations indicate a confirmation of the phenomenon. –  acmshar Jun 2 '11 at 18:27
    
@acmshar You can thank team member aarthi for relinking your account. –  dmckee Aug 10 '12 at 2:30
    
Note that while the name 'dark energy' sounds mysterious, it's really nothing more than a placeholder. We don't know what the energy fuelling the expansion is, but we do know it exists. So, until we come up with an explanation for it, it's known as dark energy. When it's discovered what dark energy is, the name will (probably) be discarded in favour of something more descriptive. –  Benjamin Hodgson Aug 10 '12 at 10:03
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In the late 1990's, astronomers were measuring the speed and distance of distant galaxies away from us, and trying to see how fast the universe was decelerating. A surprising find was made, that the expansion of the universe is accelerating, seeming to defy the laws of gravity. Study after study has been made to confirm this, and everything points to it definitely being there.

There are a few theories as to what is causing it. The most popular is that it is Einstein's Cosmological constant, except that Einstein predicted that it would make the universe stand still, not accelerate. Other theories include negative pressure, and Quitessence. For more information, see the Wikipedia article.

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Your answer seems to actually lack a portion containing a reference to dark energy, except the Wikipedia link. –  rfusca Jun 2 '11 at 2:00
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When astronomers say they are studying dark energy, they typically mean that they are exploring this anomaly. This acceleration of the expansion seems to imply an unknown energy density (hence the name dark energy), but not many astronomers are committed to specific explanations at this point. –  EHN Jun 2 '11 at 2:21
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Dark Energy is supposed to substantiate the theory of cosmic inflation; the substance itself is a hypothetical one. There is also a little controversy, though: Albert Einstein provided the idea with his cosmological constant 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 redshift 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.

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