# CMB - Excess Energy? [duplicate]

Today in physics we were looking at how the energy of a photon is the product of Planck's constant and the frequency of the photon, therefore the lower the frequency, the lower the energy of the photon.

The Cosmic Microwave Background, which I understand is a literal stretching of space over time, therefore increasing the wavelength of the radiation emitted during the big bang, being electromagnetic waves (photons), which will of course decrease the frequency.

So, if over time an individual photon's frequency has decreased, so has its energy... Where has the excess energy gone?