cosmic background radiation
The cosmic background radiation is diffuse electromagnetic radiation coming from all parts of the sky. It is made up the cosmic microwave background (CMB), cosmic X-ray background (CXB), cosmic infrared background (CIB), and cosmic optical background (COB). Of these components, the CMB is by far the largest, with a total intensity of 996 nW/m2/steradian (nW = nanowatt, or 10-9 W; "steradian" is the solid angle whose opening is one radian). The cosmic far-infrared background has a total intensity of 34 nW/m2/sr, while the cosmic near-infrared background and COB combined have a total intensity of slightly less than 60 nW/m2/sr. Together the infrared-optical backgrounds add up to about 9% of the CMB's intensity.