The Big Brain Theory presents an answer to the question: "What is the universe?" This answer is surprising, but its feasibility is demonstrated in a very didactic way and supported by too much consistent argument. In this demonstration, the reader will know a way to build and integrate all beings from the micro to the macrocosm and the mathematics' role in this. After this demonstration, the reader will know some hypotheses based on the Big Brain Theory for questions such as: "Does the supernatural exist?"; "What exists?"; "What are we?"; "Does God exist?"; "How to explain the expansion of the universe, the origin of mass, the wave-particle, the quantum entanglement, the gravitational lens and the nature's forces origin?" Warning, this knowledge acquisition can change your world view. Reading the Big Brain Theory is recommended for those who like it, have the courage and can think outside the box.