Hello all. I was watching the prof. Susskind lectures on YouTube. I'm very grateful to Stanford and the professor for making that knowledge so easily available to people willing to learn. While I did understand almost everything said I still have some question of things that I did not understand or things that were not discussed. While discussing energy dominated universe the professor said that the expansion of space causes red shift to the photons, thus reducing their energy until the universe became matter dominated. My question is where that energy disappeared? While discussing vacuum(or dark energy) dominated universe he said that the energy density of vacuum does not change with time, thus expansion of the universe creates more energy. When asked where this energy comes from the professor pointed to the other side of the equation (the one not containing the dark energy density constant). I don't understand what he was talking about. The other side of the equation then has to be unlimited source of energy. How that is possible?