The study of dark energy in this article starts with the physical mechanism of curved space, and it is found that objects can change the physical properties of vacuum ��0��0,The physical changes in vacuum ��0��0 can also have an impact on the internal energy of objects. The continuous variation of kk in space is the physical mechanism of curved space. Objects in the continuous variation of ��0��0 will spontaneously accelerate their motion in the direction of increasing ��0��0, because the internal energy of the object will be lower there,The increased kinetic energy of accelerated motion of an object comes from the reduced internal energy of the object. The macroscopic distribution of ��0��0 in the universe increases continuously from the inside out. The acceleration and expansion of the entire universe is the spontaneous motion of galaxy objects towards low-energy space. The reduced internal energy of galaxy objects is converted into the increased kinetic energy of galaxy objects. The internal energy transformed by galaxy objects is the dark energy we have been searching for. The phenomenon of photon fatigue provides strong support for the idea that ��0��0 in the universe continues to increase from the inside out.