Neutrinos interact extremely weakly (by the weak interaction), which means that one either needs quite a lot of neutrinos (which they do in terrestrial experime

Neutrinos interact extremely weakly (by the weak interaction), which means that one either needs quite a lot of neutrinos (which they do in terrestrial experiments) and/or a very large detector with a lot of mass and area. For example the ice cube neutrino observatory has a Gigaton of ice as detector mass for neutrinos to interact with. And still the event rate is only on the order of a couple hundred neutrinos _per day_. Most of which are from the Sun - which has a neutrino flux of more than 10^10 neutrinos per second per square centimeter (at the distance of Earth!).