There is a stable cosmic background photon gas fulfilled in our universe and a very weak damping force is acting on every traveling photon, then the light red shift has been observed at a large distance. When a photon travels in the cosmic background photon gas, its frequency continually lowers and at last diminishes in the photon gas as a low velocity one. The decay factor of traveling photons has been estimated; the light red shift is nonlinear in space, and our visual field is finite although the universe is infinite.