The Higgs Boson
In our macro world, we assume all particles have mass, however small. But in the micro world, electroweak theory,which ties the electromagnetic and weak forces into one underlying force, predicts that special particles called mediators should have no mass at all; which is a problem, because some of them do.
Mediators are force-carriers: Photons transmit electromagnetism, while W and Z bosons carry weak force. But while photons are massless, W and Z bosons pack substantial heft, on the order of 100 protons apiece [source: CERN].
In 1964, physicist Peter Higgs of the University of Edinburgh and the team of François Englert and Robert Brout of the Free University of Brussels independently proposed a solution: an unusual field that conveyed mass based on how strongly particles interacted with it. If this Higgs field existed, then it ought to have a mediator particle, a Higgs boson. But it would take a facility like the LHC to detect it.
In 2013, physicists confirmed that they'd found a Higgs boson with a mass of roughly 126 giga-electron volts (GeV) -- the total mass of about 126 protons (mass-energy equivalence lets physicists use electron volts as a unit of mass) [sources: Das]. Far from closing the books, this opened up whole new areas of research into the stability of the universe, why it seems to hold so much more matter than antimatter, and the composition and abundance of dark matter [sources: Siegfried].