The field of astroparticles is at the boundary of particle physics with astrophysics and cosmology. It is an extension of the discipline known as nuclear astrophysics which itself uses the laws of nuclear physics to understand the origin and evolution of stars, galaxies and the universe, and conversely it uses these studies to explore the behaviour of nuclei in extreme conditions.
At higher energy levels, various astrophysical phenomena are found to involve particle physics, and observing these astrophysical phenomena helps us to understand the properties of elementary particles.
Dark matter is a good example of this phenomenon: the masses of the elementary particles making up the hypothetical dark matter are related to the large scale structure of galaxies and galaxy clusters.
Some theories are difficult to test in an accelerator because very high energies must be reached and accelerators longer than a thousand kilometres would be needed. However, the universe comes to our aid and produces high energy cosmic rays that can be observed. To a certain extent, some of these theories can be tested by studying cosmic radiation from observatories such as Auger.