Neutrinos are a notorious thorn in physicists' sides, as the majority of them go missing while traveling from the Sun to our detectors on Earth. We're alsomiscounting their antimatter counterparts, which could mean big things for our understanding of physics.
The original neutrino mystery was solved back in 2001. Here's the short version: in the 1960s, physicist Ray Davis determined that only about a third of the predicted amount of solar neutrinos ever actually reached Earth. Decades later, Canada's Sudbury Neutrino Observatory found the missing neutrinos by realizing that there are actually three flavors of the particle, and Davis had only been detected the smallest variety, the electron neutrino. (For more background info on neutrinos, check out our field guide.)