The vacuum of space is not so empty. Apart from the plethora of photons spanning from radio to gamma rays crossing interstellar and intergalactic space, there is a large variety of massive particles with cosmogenic origin. Particle Astrophysics studies the generation, evolution and properties of these particles, which include cosmic rays, neutrinos, and dark matter.
These particles can be studies directly with detectors in space, on the ground, or underground. They can also be studied by the products of their interactions, which can happen locally with the Earth or its atmosphere, or much farther away.