Astrophysik: Fenster zum Kosmos
... are tiny elementary particles which extremely rarely interact with other matter.
They penetrate the whole universe almost unaffected, including the Earth, of course - billions of times per second.
Neutrinos can be observed only indirectly, namely by means of their rare collisions with an atomic nucleus.
During these collisions, neutrinos are often transformed into Myons, a sort of heavy electrons.