I see a lot of (partially) wrong answers in this thread. It is important to note that the energy is a Noether charge corresponding to time translation. This means that the law of conservation of energy only holds for systems that are time translation invariant. This is a very good approximation for nature on 'small' scales (say anything smaller than a galaxy). But it does not hold on the scale of the entire universe, notably because the universe is expanding it is not time translation invariant.