Which particle was detected in 1956 despite passing through a light year of lead unimpeded?

The neutrino, proposed by Pauli in 1930 as what he called a desperate remedy - energy appeared to go missing in radioactive decay, and rather than abandon conservation of energy he invented an undetectable particle to carry it away. He apologised in writing for postulating something nobody could ever find.

Reines and Cowan found it 26 years later, beside a nuclear reactor at Savannah River, by looking for the distinctive double flash of a neutrino striking a proton. They had earlier proposed doing the experiment next to an atomic bomb. Roughly a hundred trillion neutrinos pass through a body every second, and over a lifetime perhaps one interacts.

They have kept overturning things since. Detectors found only a third of the neutrinos the Sun should produce, and after decades of suspicion falling on the solar models the answer turned out to be that neutrinos change type in flight - which requires them to have mass, which the standard model had said they did not. They also arrive from a supernova hours before its light does, since they leave the collapsing core immediately while photons fight their way out, so a neutrino detector is an early-warning system for the sky.