I have learnt that the concept of Center of Mass is universal and is applicable everywhere. As per Einstien's Mass-Energy Equivalence there should be a concept of "center of energy" also? For example if I annihilate electron and positron center of mass-energy should apply. The position of center of mass-energy should not be displaced as no external force is acting on the system. So does there a concept of "center of mass-energy" exists in theories?