But even though space seems very clear and empty to us, for an electrically charged high-energy particle it is actually like making your way through a crowded train station. The light that makes up the baby picture of the universe, the cosmic microwave background, fills the universe with a kind of photonic fog. Cosmic rays interact with this fog and get slowed down fairly quickly. A particle at 1021 eV can only go for a few million light-years before it gets slowed down to energies below 1019 eV or so.
