You are oversimplifying things. Yes, slow moving charged particles (such as electrons or Helium nuclei) can be easily shielded. However, fast moving particles are much harder to shield against as they create showers of new particles (of lower energy) upon collision.
The spectrum of these particles extends way up - scientists are busily observing particles with energies on EeV scale (roughly what a moving golfball has), though these are quite rare.
Neutral particles, like gamma rays, can only be shielded by a bulk material - the penetration depth depends on density.
Lastly, we have we have direct visual observation of cosmic rays by astronauts on Apollo missions and ISS.
In summary - being in space is kinda like being on a battlefield - if your general did not screw up the chance of being hit by an artillery shell is quite small. But this does not mean it cannot happen.