When the bound 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. Mass energy (Mc^2) is an attractive component, and the equivalent mass (-M_gp) of gravitational potential energy is a repulsive component. Therefore, if |(-M_gp)c^2}| < Mc^2, there is a decelerated expansion, and if |(-M_gp)c^2| > Mc^2, accelerated expansion is performed. |(-M_gp)c^2| = Mc^2 is the inflection point from the decelerated expansion to the accelerated expansion. Dark energy is gravitational potential energy or the energy of the gravitational field. Dark energy arises because any positive energy (mass) contained within the range of gravitational interaction creates negative gravitational potential energy. While positive mass energy is proportional to M, absolute value of gravitational potential energy increases faster because gravitational potential energy is proportional to -M^2/R. Therefore, as the universe ages and the range of gravitational interaction expands, a situation arises where the negative gravitational potential energy becomes greater than the positive mass energy, and thus the universe is accelerating expansion. I present Friedmann equations and dark energy function obtained through gravitational potential energy or gravitational self-energy model. There is no cosmological constant and 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.