Astronomy observes large structures: our universe. Particle physics studies the smallest building blocks of all matter. Astroparticle physics combines these two extremes.
Our Earth is continuously being bombarded by particles from outer space. A small fraction of these particles was unexpectedly found to carry a lot of energy. To discover where such particles, and the source of their energy, come from, new particle detectors are needed that are suitable for observing neutrinos and protons with a very high energy. That is difficult because the particles concerned are very rare and, moreover, they normally fly unnoticed through detectors.
Nikhef's astroparticle research:
Cosmics (currently only in Dutch)
Neutrinos (currently only in Dutch)
Download the English brochure on astroparticle physics at Nikhef.
Have a look at this TED-Ed video about cosmic rays: