What is the official explanation for the Cosmic Background Radiation?

Both answerer's are correct, thumbs up to both of you. Just a clarification on one of your points that I think you may still be confused on. The CMBR represents our visible limit of the Observable Universe. So, if we were to look at it as a kind of boundary, then its recessional velocity would be near the speed of light, as its age is only 370,000 years after the birth of the Universe. The actual particle horizon of the Observable Universe is receeding from us at light speed, and everything outside of it is receeding at FTL speeds. But the CMBR is a remnant, therefore, there is no remaining source.