reposted from Nasa
What is Dark Energy?
We don't know. We know how much there is, and we know some of the properties it must have. Other than that, dark energy is a mystery, and it's important that we find out more.
Roughly 70% of the universe is made of dark energy. Dark matter makes up about 25%. Everything on Earth, everything that we have ever observed with all of our instruments – normal matter – adds up to less than 5% of the universe. Then again, maybe we shouldn't even continue call it "normal" matter since it's just a small fraction of the universe!
Albert Einstein was the first person to realize that empty space is not the same as nothingness. Space has amazing properties, many of which are just beginning to be understood. The first property of space that Einstein discovered is that it is possible for more space to come into existence. One version of Einstein's gravity theory makes a second prediction: "empty space" can possess its own energy. This energy would not be diluted as space expands, because it is a property of space itself; as more space came into existence, more of this energy-of-space would come into existence as well. As a result, this form of energy would cause the universe to expand faster and faster as time passes. Unfortunately, no one understands why space should contain the observed amount of energy and not, say, much more or much less.
But why haven't we observed it? Often, we measure differences, not absolute values. When we talk about the height of mountains, we are talking about how high they are above sea level, not the distance between the mountain top and the center of the earth. We assume a "floor" for these purposes. Likewise, we measure differences of energy in the universe – but there could be a "sea-level" for energy in the universe that we cannot yet measure.
What is the nature of this energy? As scientists developed the quantum theory of matter, they realized that "empty space" was full of temporary ("virtual") particles continually forming and destroying themselves. Physicists began to suspect that indeed the vacuum ought to have a distinct form of energy, but they could not predict its magnitude.
While theoretical physicists were trying to come to grips with dark energy, observational astronomers were trying to explain a bizarre result. Theories at that time predicted that the universe's mass should be slowly overcoming the momentum of the big bang, causing the expansion of the universe to slow down. But observations of supernovas in other galaxies were showing that the universe was actually expanding much faster than expected. Something was causing the universe to have another growth spurt!
Theory and observation dovetailed in dark energy. The dark energy has presumably been around since the beginning of the universe, but its effect may become more dominant as the universe expands.
We still do not know whether or how the highly accelerated expansion in the early Universe (inflation) and the current accelerated expansion (due to dark energy) are related.
This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart. Credit: NASA/STSci/Ann Feild
NASA is planning missions to find some clues to solve the mystery of dark energy. These missions will investigate the variation of the universe's density over time, and how the universe's rate of expansion has changed over time.
What About Dark Matter?
 |  Photo: NGC 4555 - this large, isolated, elliptical galaxy is embedded in a cloud of 10-million-degree Celsius gas. NASA/CXC/E.O'Sullivan et al [+ more]. |
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The image at the right is not a picture of dark matter. It is a picture of its effects, captured by the Chandra X-ray Observatory. It's a galaxy surrounded by a cloud of extremely hot gas. In order for the gas to stay around the galaxy, a halo of matter we cannot see must be holding it there with its gravity.
Astronomy relies on light, whether this light is x-rays or radio waves or the light we can see with our eyes. But most of the matter in the universe is dark: it doesn't create its own light like stars do. (We see planets and asteroids because they are illuminated by the stars they orbit. These objects are not considered dark matter.)
As with dark energy, we know very little about dark matter other than how much there is – about 25% of the universe's mass.
Dark matter can be measured indirectly, by calculating how much mass is required to create enough gravity to make galaxies behave the way they do. There are many theories about the nature of dark matter. One says that it is weakly interactive massive particles (WIMPs) another says that it is massive compact halo objects (MACHOs).
'It's hard to picture' 
