Dark energy has not been explained other than to state that it may be the driving force behind the expansion of the universe. Each topic in the evolution of the universe has its own explanation: Temperature versus time is driven by radiation, then by matter, then by dark energy; Cosmic Microwave Background (CMB) isotropy is driven by Inflation; Matter production is explained by saying sub-atomic particles get together but do not say where they came from; Recently discovered acceleration of expansion of the physical universe (that collection of things we see all around us) is still awaiting a consistent explanation. A single theory of dark energy with no adjustable parameters answers all these concerns and more. The temperature in the singularity was too great for matter to exist, thus, dark energy is a massless form of energy. It produces matter by the Breit-Wheeler process. Dark energy is the remains of the total energy from the singularity after matter production ceased. It is a perfect fluid and expands adiabatically and homogeneously with large initial velocity and will be analyzed by Friedmann’s solution of Einstein’s Field Equations. It forms a homogeneous sphere that keeps temperature, pressure, and matter isotropic. The physical universe expands at a rate that is the difference between the expansion due to dark energy and the inward rate induced by gravity. This accounts for the increased acceleration of distant cosmological entities. The expansion slows but never stops. The CMB radiation is the thermal footprint of dark energy.