The origin of cosmic rays
Cosmic rays are secondary particles
Most of the cosmic rays that hit the Earth are very fast moving protons emitted from the Sun and other stars. They collide with the nuclei of air molecules in the upper atmosphere causing a shower of secondary particles.
Animation showing how cosmic rays are secondary particles formed in the upper atmosphere by protons from deep space.
One of the more common is a type of heavy electron called a muon. So the background cosmic radiation is really the shower of secondary particles formed in the upper atmosphere.