So what is dark energy? Well, the simple answer is that we don't know. It seems to contradict many of our understandings about the way the universe works.
We all know that light waves, also called radiation, carry energy. You feel that energy the moment you step outside on a hot summer day.
Einstein's famous equation, E = mc2, teaches us that matter and energy are interchangeable, merely different forms of the same thing. We have a giant example of that in our sky: the Sun. The Sun is powered by the conversion of mass to energy