We postulate that our space is filled by the “Gamma elements” having energy and mass with the size approximately given by the wavelength of the highest energy gamma rays and that time and distance are both discretized by the process of light propagation from one Gamma element to the next. These postulates provide us with a theoretical ground to explain why the speed of light, c, should remain constant to all observers regardless of their inertial frames of reference. In the cosmological scale, the energy of the Gamma elements filling space has been equated to the energy represented by the cosmological constant, i.e., the dark energy. When applied to the expanding universe, the EST model brings additional 25% of the total mass simply as the relativistic correction to the non-relativistic results; hence the dark matter is closely identified as merely the relativistic correction to the dark energy predicted by the Friedman equation. The agreement with the observed magnitudes convincingly supports this interpretation of dark energy and dark matter.