NASA's COBE satellite
The Cosmic Microwave Background (CMB) appears at the limit of vision - we look back in time to see the cosmic "fireball". A contrast stretched image reveals slight patchiness arising from slight density variations.
The DMR instrument on COBE gave an exceedingly accurate spectrum (shown here with error bars amplified by ×400). It is a perfect blackbody, with temperature 2.728 K.