Disclaimer: I am not a physicist and this was just some random question that popped into my head. According to the law of conservation of energy, energy cannot be destroyed or created. So the energy in our current universe is the same as it was right after the big bang right? But cant gravity add more energy in a universe thru potential/kinetic energy? Another way of looking at this: imaging an isolated universe with two grains of sand. Each grain of sand has 100% same amount of atoms, neither of them has any movement speed in any way. Yet each is separated by a distance that it would take gravity a millennium to move them to the point where they eventually orbit each other. If you were to then examine this universe a millennium after its creation, it would appear that it would now have more energy than when it was created.