In much simpler terms dark energy is a repulsive force that causes universe to expand rather than to come together due to gravitational force of attraction.
Often admired as father of the modern cosmology, Edwin Hubble, in 1929 was studying on exploding stars often called as supernovae and he observed that universe is expanding, which seemed to be contradicting with theories that suggested that expansion of universe is slowing down due to attractive force of gravity that pulls the matter altogether.(We are probably the last generation to have been taught that ‘gravity always attracts’).
In the year 1998, Hubble space telescope observations of exploding stars showed that expansion of universe was slow long time ago then it is today. Universe was expanding at a much faster rate then ever before. This came in as a surprise to astrophysicists out there. Something was there which was responsible for this phenomena so the researchers named this concept as Dark Energy.
Based on rate of expansion of universe, researchers have found out that roughly 68% of the entire universe is dark energy, 27% is dark matter(another interesting concept) and remaining 5% of it exists in the form of everything that we observe a.k.a. matter(Yes, only this much). Though researchers don’t have the exact idea of dark energy, but they have created some models that will explain everything from the inception of universe to present time and out of those models some predicts that dark energy will rip apart everything that is there in existence(scary isn’t it).
There are plenty of explanations for dark energy, out of the lot there are few which seems plausible. According to one explanation, dark energy is one among many properties of space. Albert Einstein was the first one to realize that empty space is nothing. According to a prediction that was made by Einstein’s gravitational theory that discusses about cosmological constant, empty space possesses its own energy and being a property of the space it does not get diluted, rather more and more of this energy gets created as universe expands.
Another explanation brings into picture the concept of Quintessence(buzzword for today:)), the word means ‘fifth element, after Earth, Water, Air and Fire proposed by ancient Greeks to describe a perfect, sublime substance. In cosmological world the four essences are normal matter, radiation (photons), cold dark matter, and neutrinos so dark energy can be described as Quintessential element.
Another idea suggests that dark energy is an additional force that is joining the queue with four already known fundamental forces (gravitational, electromagnetic, weak nuclear and strong nuclear force) but being a fundamental force it is not experienced by us muggles(Yeah!! Harry potter reference) so astrophysicists have built creative models that suggest that this force is hidden.
There are tonnes of other explanations as well but none of them have been proven by astrophysicists, so there is plenty to be discovered in regard to dark energy and astrophysicists will continue to do so because eventually it is key to understand the universe and its fate.
Dark matter and dark energy are two things we measure in the universe that are making things happen, and we have no idea what the cause is-Neil deGrasse Tyson.