Dark energy and dark matter constitute about 95% of the Universe. Nonetheless, not much is known about them. Existing theories, including General Relativity, fail to provide plausible definitions of the two entities, or to predict their amounts in the Universe. The present paper proposes a new special relativity theory, called Complete Relativity theory (CR) that is anchored in Galileo’s relativity, but without the notion of a preferred frame. The theory results are consistent with Newtonian and Quantum mechanics. More importantly, the theory yields natural definitions of dark energy and dark matter and predicts the content of the Universe with high accuracy