A relativistic theory of cosmology proposing that the size of an observers Universe depends on it's own rest mass, and that the observers total potential energy domain is 2mc² , spanning the range from -mc² to +mc² . The space dimension of the observers Universe is shown to be limited by the Schwarzschild Radius near the observers center of gravity and by the observers Mass Horizon Radius (MHR) which is the maximum distance from the origin, that an observer of mass m is able to interact. By a leap of faith, this theory draws the conclusion that the size of the Universe is a mass dependent variable. Calculations show that the current astronomical measurements of the Universe, agrees closely with the MHR for a human weighing around 80 kg. More importantly, the MHR gives us a mass to space ratio, which allows us to accurately calculate the mass-energy of empty space.