Occam's razor. If you postulate that there are things that don't interact at all with reality as we know it, that's a more complicated theory that has exactly the same predictive value as the theory that contains no such things.
What is it? It's not matter. It's not energy. Perhaps we could say that it's an electrochemical pattern that exists in the brains of a few million people. But if we all died would a non-material thing named "Occam's Razor" exist? If everyone who knew what Occam's Razor died, and then a hundred years later some one read about it in a book, would it spring back into existence?
You can view things like abstract concepts as information. You can create information by using energy and e.g. encode it onto a surface or encode it in a human brain. There is a minimum amount of energy necessary to store information defined by Landauer's principle for irreversible computation. So in that sense the idea of "Occam's Razor" really does exist encoded in your brain as information (a form of energy) manifested in connections between neurons or so.