Cosmic background radiation
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
, etc., with different causes; most of these are ultimately attributable to unresolved individual sources.