  The most abundant particles in the Universe are photons and neutrinos. Both
types of particles are whirling around everywhere, since the early Universe.
Hence the neutrinos are all around us, and permanently pass through our planet
and our bodies, but we do not notice: they are extremely elusive. They were
suggested as a theoretical hypothesis in 1930, and discovered experimentally in
1956. Ever since their properties keep on surprising us; for instance, they are
key players in the violation of parity symmetry. In the Standard Model of
particle physics they appear in three types, known as "flavors", and since
1998/9 we know that they keep on transmuting among these flavors. This
"neutrino oscillation" implies that they are massive, contrary to the previous
picture, with far-reaching consequences. This discovery was awarded the Physics
Nobel Prize 2015.
