Atomic nuclei (mostly protons) and electrons that are observed to strike the Earth's atmosphere with exceedingly high energies.
The energy spectrum for cosmic rays
Cosmic rays are energetic charged subatomic particles, originating in outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation. Most primary cosmic rays (those that enter the atmosphere from deep space) are composed of familiar stable subatomic particles that normally occur on Earth, such as protons, atomic nuclei, or electrons. However, a very small fraction are stable particles of antimatter, such as positrons or antiprotons, and the precise nature of this remaining fraction is an area of active research.
Source: wikipedia, NASA