Everything (or every thing), is all that exists; the opposite of nothing, or its complement. The totality of things relevant to some subject matter. Without expressed or implied limits, it may refer to anything. The Universe is often defined as everything that exists. It may refer to an anthropocentric worldview, or the sum of human experience, history, and the human condition in general.
Theory of everything 
In theoretical physics, a theory of everything (TOE) is a theory that would explain and unify all known fundamental interactions of nature in a single model. Originally, the term was used playfully in regard to overgeneralized theories. However, it became a popular way of referring to the “holy grail” of physics. A theory of everything in philosophy would ask: "Why is reality understandable?" "Why are the laws of nature as they are?" "Why is there anything at all?" The idea of everything is simply an answer to lost questions, a reason for reasoning, and the idea that all is simply all, as everything is everything.