In order to apply general relativity to a strong gravitational field, the gravitational self-energy of the object itself must be considered. By considering the gravitational self-energy, it is possible to solve the singularity problem, which is the biggest problem with general relativity. When an object acts on gravity, the gravitational action of gravitational potential energy is also included. Therefore, even in the case of the universe, the gravitational action of gravitational potential energy must be considered. Gravitational potential energy generates a repulsive force because it has a negative equivalent mass. For the observable universe, I calculated the negative gravitational self-energy, which is approximately three times greater than the positive mass energy, which can explain the accelerated expansion of the universe. The source of dark energy is presumed to be due to gravitational self-energy and an increase in mass due to the expansion of the particle horizon. It is calculated that the effect of dark energy is occurring because matter and galaxies entering the particle horizon contribute to the total gravitational potential energy. While mass energy is proportional to M, gravitational self-energy increases faster because gravitational self-energy is proportional to -M^2/R. Accordingly, an effect of increasing dark energy occurs. I present Friedmann's equations and dark energy function obtained through gravitational self-energy model. There is no cosmological constant, dark energy is a function of time. This model predicts an inflection point where dark energy becomes larger and more important than the energy of matter and radiation.