Primordial Resonant Lines and Observational Strategy for ODIN and HERSCHEL

The presence of a primordial molecular or atomic medium can give us the possibility to investigate the first phases of the structure formation. The lines produced by resonant scattering of the CMB photons are the most important signals coming from the dark age of the post-recombination universe. The possibility to detect these lines with ODIN and Herschel satellites are investigated.