The quark model of matter is on the most successful models in modern physics. It explains the structure of the protons and neutrons in matter, it explains why we see quarks in mesons and baryons, and it predicts a “periodic table” of hadrons. It was all going so well until the X(3872) came along and messed everything up. When Rabi heard the news of the discovery of the muon he replied “Who ordered that?!” It’s happened again, this time with hadrons.
[Blog post] Who ordered that?! An X-traordinary particle13 10 2011