Let's eavesdrop on an imaginary debate about energy.
Unreal: Energy is not real. There is no stuff called energy.
Real: Of course energy is real! You can measure it. A gram of fat has more energy than a gram of sugar. If it wasn't real, how could we talk about the amount of energy in a system? Also, objects can pass energy from one to the other. The Sun can give a green plant some energy. I can give a baseball some kinetic energy by batting it into the outfield. A spring can store potential energy. Doesn't that sound real to you?
Unreal: No! Energy is just an accounting tool that scientists use to make sense of such things as motion.
Real: Einstein's famous equation "E = mc²" says that you can convert mass and energy back and forth into each other. If energy isn't real, then mass can't be real either.
Unreal: It depends on what you mean by "real." There is no single thing that is energy. Mass is one manifestation of what we call energy, but so is height above a table. How real is that?
Real: Look, I don't have the energy to argue anymore. I don't think we will ever agree.
Unreal: Ok, ok. Maybe later.
There are a variety of opinions. Some experts urge educators not to use term like "flow of energy" because they imply that energy is real. Others assert that mathematics is the only valid language for discussing energy. Some, however, are sure that energy is as real as matter. Finally, some experts feel that it doesn't matter whether or not energy is real, as long as you can use energy to describe and predict interactions with precision.