10 billion years ago, photons emitted at the surface of last scattering were being absorbed by whatever stuff was around. Those atoms would absorb those photons and release other photons, but these would not be primordial photons from the CMB anymore but photons emitted at a later time by the stuff existing at that later time. The same 9 billion years ago. The same 6 billion years ago, The same 2 million years ago and the same in the XIX century. We are more than 13 billion years from the surface of last scattering and we are still detecting photons from the CMB. It seems reasonable to believe that if humanity will stay alive, it will keep on detecting photons from the CMB for many centuries or millennia to come. How come they have not all been absorbed by some atom in all this time? Were there indeed so many photons released that no matter how much stuff there is to absorb them, there are still plentiful of them to keep on traveling forever to every edge of the Universe? Is it conceivable that at some stage an hypothetical civilisation would not be able to detect the CMB anymore because all of its photons had been absorbed by the stuff 'in between'?