You know that our universe is expanding at an accelerating rate. Dark energy is the name given by scientists to this expansion generating force. Dark energy can be described as the reverse of gravity. While gravity pulls things together, dark energy pushes things away.
An accidental observation in 1998 led to the hypothesis of dark energy. During a study, astronomers observed that supernovae under observation were dimmer than anticipation. This meant both that those supernovae were farther away and that universe was expanding faster than envisaged. Dark energy was the most accepted hypothesis to explain the observations.You may be surprised to know that many things about the nature of dark energy still remain to be identified.Scientists are still performing experiments to know whether the theory of relativity or general laws of physics is applicable to dark energy.
As of now, it is estimated that 68.3% of the total energy in the universe is contributed by dark energy.