We know that the universe has been expanding since the Big Bang explosion. This expansion exists until now. The amount of gravitational force in the universe is supposed to slow down the expansion of the universe. But it's not happening. Why is this happening? Scientists haven't found anything that suggests the expansion of the universe is speeding up rather than slowing down. But they could easily understand existence of the energy behind it. This is not supposed to happen without any Energy. Scientists call this unknown energy Dark Energy.

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It was certain that the universe was expanding in the early 1990s. Generally we know that in an explosion the object has a high velocity at the beginning and this energy decreases with time. But the opposite is happening in the expanding universe. In 1998, the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today.

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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

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It surprises scientists. No one knows about it who will describe it. But there is definitely a reason behind this happening. None of the theories explain this. But Albert Einstein's theory says that space is not just empty space. But his theory of gravity does not describe this cosmic acceleration. Especially new and old theories don't give a proper idea about this. So scientists came up with a new theory and named it dark energy.

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There are many mysteries in the universe that we don't know about. We know very little about the universe. It indicates that we humans have a higher perceptual capacity than other animals but its range is very small. How many percent of the universe have humans known so far?

It is easy to understand that an enormous amount of energy is causing the universe to expand. Even if we do not know about this energy, we can tell by its effects that there is hidden energy which is unknown. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the universe.

If you read the part linked above, you can understand the amount of dark energy. Most of our universe is dark energy. Where did this amount of dark energy come from? The answer is the big bang. Again 27% dark matter is in our universe. The matter we can see planets, stars, nebulae etc is only 5%. What a strange universe. There is no other way but to put the eyes on the forehead. We have no existence relative to the universe and visible matter in the universe does not exist relative to dark energy and dark matter. That is, we and visible objects like planets, stars, nebulae etc are very small part of this universe. After knowing all this information, heart attack is wanted. Ha ha ha ....

However, Albert Einstein realized that space is not empty space, it has many properties. Albert Einstein theory suggests that. But his theory cannot solve the dark energy puzzle. Albert Einstein makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe.

Anyway, any of the other theories that exist so far do not give a correct idea about dark energy. At present it remains a mystery. If any correct theory comes then that theory is going to get Nobel Prize. A correct explanation of dark energy will enrich our world of science many times more.

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This post was written with information from NASA's website