Energy is a mathematical quantity - not a kind of stuff. When physicists talk about energy "transforming from one form into another" they are speaking in a kind of lazy shorthand. I like to make the analogy that energy is like "value". It's a number that we associate with objects. A dollar bill and a quarter both have value. So does an ice cream cone. If you buy a 50 cent ice cream cone with a dollar bill and get two quarters in change, your dollar didn't "transform" into an ice cream cone. The "value" of your dollar didn't change shape or move somewhere. There was an exchange in the physical world that we can account for by using the abstract quantity we call "value". You exchanged a piece of paper, which has a "value" of 1 for an ice cream cone which has a value of .5 and two shiny metal disks that each have a value of .25.
When physical objects in the universe interact with one another, we can do an accounting of these interactions using an abstract quantity we call "energy". But energy is not any sort of "extra thing" in that physical process.
Also - a great video from astrophysicist Matt O'Dowd (assistant professor at the Physics and Astronomy Department at the Lehman College of the City University of New York. https://m.youtube.com/watch?v=PUn2izowBkw
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