In outer space there is a type of very high energy radiation called galactic cosmic rays. These rays are comprised of charged particles, like atomic nuclei and electrons. They are one of the two major contributors to the radiation dose received by astronauts (the other coming from solar particle events), and the only one that it has been impossible to shield against thus far.
These galactic cosmic rays are ubiquitous in outer space.
While they are never, or very rarely, mentioned during Star Trek, they are a very important part of travel in outer space, and it is vital that astronauts and engineers know about them before leaving the safety of the earth's atmosphere.
