A proton and an electron in a hydrogen atom experience electric and gravitational forces on them.If a photon is absorbed the electric and gravitational forces decrease.But suppose the sum of all forces in a region of space-time stays constant:if the electric and gravitational force decreased then perhaps another force- a new force, so far unknown to science - increases, so the sum of all forces is the same as it was before the electric and gravitational forces decreased. This means that the electric and gravitational force carriers would have to communicate with the force carriers of the new force field.And the same would apply to the colour force and weak force.But the new force would have to have no effect on the trajectories of electrons, protons etc. or else forces would not be observed to change with distance -forces would be constant. So how would the new force influence the physical world? If we consider the momenta of force carriers as representing a force, then as the momenta of the electric force,for example, get smaller as the electric force weakens with distance,the momenta of the new force carriers would get larger.The electric force carriers would transfer energy to the new force carriers. The new force carriers must be exerting their effects not on protons and electrons and other leptons and hadrons but on space itself! Just as electric force carriers push electric charges apart, the new force carriers could push regions of space apart.In other words, the new force which keeps the sum of all forces in a region of space constant, is dark energy.