A radiation that is an after-effect of the Big Bang still around today. It was actually discovered accidentally by two Arno Penzias and Robert Wilson in 1965. They noticed a static that appeared to be coming from every direction at the same strength. Later scientists were able to trace out a Black Body curve for this background radiation, shown below.
Essentially, at very early point in the beginnings of the universe, everything cooled down enough to where atoms could form and this radiation was formed. The temperature to which it has cooled, estimated to be around -270 K, scientists believe to be consistent with a universe that started at a very high temperature and in a very dense state, the Big Bang. Therefore, this cosmic radiation is very strong evidence in favor of the Big Bang.