There is one missing piece of the Standard Model - the Higgs Boson.
The Higgs mechanism is an intricate part of the Standard Model that gives rise to particle masses. It also produces one extra particle known as the Higgs Boson, which has not been found to date.
The Large Hadron Collider at CERN is designed to find the Higgs. Particle physicists are also looking to the LHC for clues of what physics lives beyond the standard model.
COSMOS to HUMANITY, Spring 2008
Lecture 2 , slide 26