Dark Energy is a force that is repulsive in nature, which physicists believe to exist throughout the universe, and the evidence for has moved from the circumstantial to the likely. Just as with dark matter, where although it can't be directly observed, its effects can be seen. In this case the Wilkinson Microwave Anisotropy Probe has made observations on the largest scale possible, the entire universe, that indicate dark energy is actually needed to explain what we see.

It all started when the Department of Energy's Lawrence Berkeley National Laboratory, and the High-Z Supernova Search team in Siding Spring Observatory in Australia had been running a project in 1998, measuring the distance to certain kinds of supernova (type Ia), in an attempt to in turn calculate the rate of expansion (or contraction) of the universe. They expected to find the the rate of expansion was in fact slowing, what they actually found was completely unexpected, the rate of expansion had been increasing!

So far the most succesfull model of the universe is the Big Bang theory, describing a universe that expanded from a hot, dense, (arbitarily) small point. The rate of expansion after any cosmological inflation is determined by the mass/energy of the universe, and the curvature ¹ of the universe. If there is more than a critical density, the 'explosion' of the big bang will blow it up to a certain size, before gravity stops it and it begins to contract again, at an ever increasing rate. If there is less, nothing will halt the expansion and the universe will carry on growing, at a rate that decreases over time; settling to a constant positive value. Until recently it was thought that the energy density, 'Omega' of the universe is exactly one, corresponding to a flat universe where the rate of expansion slows down continually until the expansion is so slow it appears to be static. To make it come out as one, admittedly 'dark matter' has to be added to the mix, which was at first a strange concept, but not totally unexpected. Which is why the results of observations were so perplexing, no model of the universe predicted an expansion that increased with time.

To give a bit more detail, their initial results showed that for supernovae of a given redshift, its brightness is less than calculated assuming the expansion were slowing. Not to difficult to deal with, maybe the universes negative curvature could explain that, or maybe dust clouds or some other un-accounted for effect relying on 'traditional' physics could cut down the novas brightness. However, detailed analysis, and further observations strengthened the hypothesis that the universe is expanding, and not only that, but at seemingly an ever increasing rate!. Some new repulsive effect, radically different from anything physicists expected, had to be at work; this new effect whose nature remained unknown began to be termed 'dark energy'.

The case for dark energy was strengthened by observations made by the Wilkinson Microwave Anisotropy Probe, whose detailed measurements of the changes in the temperature of the cosmic fireball suggested the universe has a flat geometry; meaning there is exactly enough 'stuff' (whatever it may be) for the expansion to slow down to zero. Hang on you might think, this result contradicts the supernova one! Well yes, it does but not all the facts are in place, nor all the questions answered. The last question has to be 'What does the universe actually consist of?'

Astronomers have of course been answering this question for thousands of years; and have built up an accurate measure of the amount of material tied up in stars. Recent detailed observations of the rotation speed of galaxies and gravatational lensing by super-clusters have now put an upper bound on the amount of non-luminous 'dark matter'. But none the less a conservative estimate says that 65% of the mass needed to make the universe flat is still missing.

All the pieces of the puzzle are now in place :-
1. Supernova brightness show the universe to be much larger than thought, and expanding faster and faster.
2. The spectrum of the cosmic background radiation prove the universe to be flat.
3. Measurement and calculation of the mass present to give a flat universe can only account for (at the absolute most) 35% of the necessary value.

What can bring these facts together is a force similar to gravity; in that it works over large distances and is quite weak, but unlike gravity in that it is repulsive. It now all ties up, the mass assiocated with the 'dark energy' (E=MC²) is enough to make the universe flat at the big bang, but unlike gravity, it's blowing the universe apart. The latest observations tend to put the amount of dark energy as constant over space and time, rather that as a varying 'scalar' field known as quintessence. But it might just be that Einstein was correct with his first description of the universe, which included a 'cosmological constant' working against gravity, necessary to keep the universe how he thought it was, static. What dark energy is, (like dark matter) nobody knows; but of course theories abound, further detailed observation of the background radiation should be able to prove some of them wrong.

The end is nigh!

So, taking the properties and amount (whatever it is) of dark energy into account, this is how our universe will end, far down the river of time....The matter in the universe will cool and thin, and as it spreads out new stars will be less likely to form. The galaxies will move farther apart, and the skies will dim, in fact as the expansion accelerates more of the universe will become causally disconnected. That is to say you'd have to go faster than the speed of light to ever reach parts of the universe, which is impossible. Matter itself will decay into elementary particles, which due to dark energy, will fly apart at an ever quickening rate.

Black Holes may provide a haven for a time, the massive concentration of matter in them can be harnessed for energy production. Eventually Hawking Radiation will cause these to evaporate into elementary particles, which will spread out leaving the universe utterly dark and cold. Heat won't be transferred, as the particles at some point will be so far apart; light won't be able to move between them. This is all the fault of dark energy....

As time goes on, the evidence for dark energy mounts; the first results from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) suggested that the universe is 'flat', 13.7 billion years old and that a massive 73% of the universe is 'dark energy', 23% dark matter, leaving only a froth of 4% ordinary matter.

In 2004 published studies of galaxy clusters by the Chandra X-ray space telescope, point towards the fact the expansion rate of universe stopped slowing down 6 billion years ago and has been speeding up ever since. Taking all the various results, the distribution of of 'stuff' breaks down as follows :-

1. 75% Dark Energy
2. 21% Dark Matter
3. 4% Visible Matter

It really is looking like the universe is doomed to end in a 'Big Rip'

Thanks to tdent and unperson for technical advise and corrections! You spot any more, msg me, any questions you can try me!

1. If the universe has positive curvature,corresponding to an Omega value greater than 1, then it could be a 4 dimensional sphere, where the angles of a triangle will add up to more than 180^o. Zero curvature (Omega=1) means a flat universe, with 180^o triangles, and a negative universe (omega<1)looks like a saddle and the angles of a triangle add up to less than 180^o.

Einstein predicted the existence of Dark Energy but later called it the greatest mistake of his life.

New evidence from the observation of a supernova by the Hubble Space Telescope, whose light is just now reaching us after having exploded 11 billion years ago, suggests that Einstein was right once again. In short: the light from the supernova is dimmer than would be expected and suggests that the universe has recently increased its rate of expansion. Here is an excellent analogy from a CNN.com article: "The new stellar explosion has helped astronomers understand how the universe expands, 'much the same way a parent follows a child's growth spurts by marking a doorway,' said Hubble scientist Adam Riess, lead researcher in the new study." The universe's "growth spurts" could be explained by Einstein's Dark Energy theory.