Energy is central to existence. But what is it exactly? Energy has diverse forms and is conserved in ideal closed systems...

"It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity and when we add it together it gives “28″—always the same number. It is an abstract thing in that it does not tell us the mechanisms or the reasons for the various formulas." -- R. P. Feynman.

Relevant Reference Link:

https://en.wikipedia.org/wiki/Energy