What is the nature of physical energy?

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Cathal O’Connell writes: "Any physics? textbook will tell you?energy?is?the capacity to do work. Then it usually goes on to explain that “work” is the action of?moving something against a force. But isn’t this definition kind of unsatisfying? ..."

The reason it is so hard to define is because it’s an abstract notion. In physics, the concept of “energy” is really just a kind of shorthand, a tool to help balance the books. It is always conserved (or converted into mass) so is incredibly useful in working out the results of any kind of physical or chemical process. ..." *

“The more basic is a physical notion, the more difficult to define it in words.”? -- Lev Okun?*

The Universe is observed to be accelerating, but the reason is conventionally unknown and labeled "dark energy" or the "cosmological constant" so that general relativity may conform to this observation. It has been discussed that a scale-invariant locality form of Newtonian gravitation is fundamentally repulsive, and only apparently attractive below the scale of galactic superclusters, accounting as well for "dark matter." If this is the case, the Universe is undergoing accelerated expansion because of the gravitational energy stored in space that is being released with the expansion. The three other forces and fundamental elementary particles were derived from Newtonian gravitation and special relativity at the sub-nucleon scale. Then the nature of energy might be gravitation. If gravitation and acceleration are always equivalent (identical), the nature of energy might also be described as an inertial effect of the acceleration of the Universe.

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* https://cosmosmagazine.com/science/physics/what-is-energy/

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