The Theory of Existence is the Grand Unified Theory of Everything—a single, cross-domain, empirically-validated theory explaining all phenomena. The Theory of Existence operates through the five first principles of existence—definedness, complexity, stability, recursion, and propagation—capturing all aspects of the behavior of existence… What it is, How it works, and Why it exists. The Theory of Existence contains three new theorems that are objectively true and functionally independent: 1) The Theorem of Undefined Emergence, which dictates Why existence emerged and continues to exist, 2) The Theorem of Recursive Harmonics, which dictates How existence works, and 3) The Theorem of Constants Co-Derivation, which dictates What existence is. These three theorems form The Equation of Existence—the universal law governing everything that has existed, currently exists, and can exist. The three theorems and their alignment into The Equation of Existence form the complete mathematical architecture of existence.