How can we see cosmic background radiation?
The cosmic microwave background was discovered when scientists were looking at portions of the sky from which there should have been no radiation and yet found it.
When astronomers mapped out the known universe they found that there were large spaces of nothing between all the visible galaxies. By their very nature galaxies of course generate a huge amount of radiation which we can easily detect. But when the looked into those dark places they still found radiation, not nearly at the same level as that of a galaxy but still there. They reasoned this radiation to be the echo of the big bang, the radiation left over from the time of the big bang.