This may seem like a silly question, but it has me puzzled. Theory tells us that as space expands then photons (and I guess all form of energy?) in that space are red-shifted, ie their wavelength increases, ie they lose energy. The most obvious example of this is the CMB, which consists of maybe 10^88 photons (in the observable universe) "left-over" from the Big Bang. At the time of decoupling from matter (300,000 years after the Big Bang?) each of these photons was of very high energy (low wavelength), but as the universe (space) expanded each of these photons was red-shifted (lost energy), to end up as the microwave background that we see today. If the first law of thermodynamics is to be obeyed (mass/energy must be conserved), what happened to the energy that these photons lost?