According to recent observations, the universe seems to be expanding at an accelerating rate. Theorists still don’t know what the reason is. But they have given the solution a name: Dark Energy. This acceleration of the Universe was quite unexpected. Einstein's theory of general relativity predicts that the expansion should be slowing down. To describe the missing physics needed to explain this mystery, the term "dark energy" was coined.
Roughly 70 percent of the universe is Dark Energy and 25 percent is Dark Matter. The remaining 5 percent includes everything else that all our instruments have observed in the Universe.
Why should I be aware of this?
- The nature of dark energy remains a mystery to us, and so understanding dark energy is one of the major challenges facing cosmologists and particle physicists today. 
- Dark energy is unlike gravity in that it repels matter and therefore causes the expansion of the universe to accelerate.
- It turns out that dark energy profoundly affects the lives of stars and how galaxies interact.
- Dark energy is distinct from dark matter, which is responsible for the formation of cosmic structures such as galaxies.
All about Dark Energy
Only a decade ago, light from distant exploding stars showed us that the universe's size is increasing at a faster and faster rate. Leftovers created 400,000 years after the Big Bang, called the Cosmic Microwave Background (CMB), also show evidence of universe-wide acceleration.
In end 2008 a team of researchers have analyzed 85 galactic clusters, to determine the way dark energy influenced their evolution. They found that over the past five billion years dark energy has had a profound effect on the clusters' structure, significantly slowing down the rate at which they collapse.
Why it is believed to exist?
There is no proof that it exists, but is strongly believed to exist as it is universally agreed that:
- The universe had a beginning
- The universe is expanding
- The known mass-energy of the universe cannot account for the supposed rate of expansion.
- This rate of expansion is accelerating
But, despite thousands of papers published on the subject, there is literally no experimental evidence for any of them.
Galaxy growth stopped 5 billion years ago
NASA’s Chandra X-Ray  Telescope provided compelling observational evidence that the growth of large clusters of galaxies essentially stopped about 5 billion years ago when the universe began to accelerate under the increasingly strong push of Dark Energy.
The clusters, which grew over time, were forced into full maturity about 5 billion years ago because Dark Energy kicked into high gear and started to accelerate the universe. The universe was flying apart too fast to gravitationally pull material together to grow even larger galaxy clusters.
Though this “arrested development” affects the large-scale structure of the universe it does not inhibit the ongoing formation of stars, planets or the onset of life within a galaxy.
One of the greatest challenges in science
Though things are still in the realm of conjecture, precise measurements of dark energy's properties could give us some inkling of what might be in store for the universe's future. What we know now suggests that the universe's expansion isn't likely to slow down, trigger a contraction and an eventual "Big Crunch." What's more likely is that expansion will continue at an ever accelerating rate. Hence the next few years promise to be some of the most exciting ever when it comes to Dark Energy -- one of the greatest challenges in science we have ever encountered.
The quest to understand dark energy is a major focus for NASA and the Department of Energy. Together they're funding a satellite -- the Joint Dark Energy Mission -- that is expected to increase by 10-fold the precision of the dark energy equation of state within 10 years. It will do this by measuring the properties of dark energy using both cosmic distance and the growth of cosmic structures. 
Billions of years from now, the scientists say, local superclusters of galaxies will also disintegrate and all other galaxies will ultimately disappear from the Milky Way's view. 
More on Dark Energy
What can I do to help
- What Is Dark Energy?
- The so-called mystery of Dark Energy
- The Wide Angle: Far Future Forecast - Deep Darkness