The big bang model predicts that the cosmic background radiation will be thermalized - it will have a blackbody spectrum. The measurements of the antenna temperature of the radiation at various frequencies between 1965 and 1990 had shown that the spectrum was approximately blackbody but there were some measurements at high frequencies that seemed to indicate an infrared excess - a bump in the spectrum that was not easily explained. In 1989, NASA launched the COBE (Cosmic Background Explorer) satellite to investigate the cosmic microwave and infrared background radiation. There were three instruments on board. After one year of observations the FIRAS instrument had measured the spectrum of the CMB and found it to be a blackbody spectrum. The most recent analysis of the FIRAS data gives a temperature of 2.725 ± 0.002 K (Mather et al. 1999).
A CMB of cosmic origin (rather than one generated by starlight processed by iron needles in the intergalactic medium) is expected to have a blackbody spectrum and to be extremely isotropic. COBE FIRAS observations show that the CMB is very well approximated by an isotropic blackbody.