When any patch of the
sky is observed where no individual sources can be discerned,
and the effects of the interplanetary dust, and
interstellar matter are taken into account,
there is still is
radiation.
This radiation is known as
Cosmic Background Radiation.
The origin of this radiation depends on the region of the
spectrum that we are observing.
Certainly the most famous component is the
Cosmic microwave background radiation,
a remainder of the
epoch when the
universe, still hot, became transparent for the first time to radiation.
There is also background radiation in the
infrared,
x-rays, etc., with different causes; most of these are ultimately attributable to unresolved individual sources.
See also:
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