Dark energy is an unknown form of energy which is hypothesized to permeate all of the space. It tends to accelerate the expansion of the universe.

So, we basically don’t really know what is it.

In 1929, American astronomer Edwin Hubble studied exploding stars known as supernovae to determine that the universe is expanding. Since then, scientists have sought to determine just how fast. But with our knowledge of that time, it seemed obvious that gravity, the force which draws everything together, would put the brakes on the spreading cosmos.

In the 1990s, two independent teams of astrophysicists, again, turned their eyes to distant supernovae to calculate the deceleration. However, this was the time that would change our perspective. Surprisingly, they found that the expansion of the universe wasn’t slowing down. In fact, it was speeding up! So, they concluded that something must be counteracting gravity and that something was named by scientists “dark energy.”

Changes in the Rate of Expansion over Time

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

What could cause the Dark Energy?
Maybe dark energy results from weird behavior on scales smaller than atoms. Quantum mechanics, allows energy and matter to appear out of nothingness, although only for the tiniest instant. The constant brief appearance and disappearance of matter could be giving energy to otherwise empty space.

It could be that dark energy creates a new, fundamental force in the universe. Something that only starts to show an effect when the universe reaches a certain size. Scientific theories allow for the possibility of such forces. The force might even be temporary, causing the universe to accelerate for some billions of years before it weakens and essentially disappears.

Dark energy is thought to be very homogeneous and not very dense. Also, it is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied — roughly 10−27 kg/m3 — it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such a profound effect on the universe, making up 68% of universal density, in spite of being so rarefied is that it uniformly fills otherwise empty space.

In the Friedmann–Lemaître–Robertson–Walker metric, it can be shown that a strong constant negative pressure in all the universe causes an acceleration in universe expansion if the universe is already expanding, or a deceleration in universe contraction if the universe is already contracting. This accelerating expansion effect is sometimes labeled “gravitational repulsion”.