Supersymmetry holds that all the subatomic particles we know have counterparts that are almost exactly the same, only much, much heavier. But the Large Hadron Collider hasn't found any supersymmetric particles yet, and they're running out of places to hide. Will we have to come up with a new model for subatomics?
The theory of supersymmetry is a favorite of many physicists because it elegantly explains a lot of basic mysteries about the subatomic world. The larger, unstable particles are responsible for the quantum fluctuations that would otherwise force the particles we're familiar with to be much, much more massive than they otherwise are. (This is a short version of the problem - for a more detailed explanation, check out this Nature article, or check out our own primer on undiscovered particles for more on the so-called sparticles.) Some of the supersymmetric particles could also be responsible for dark matter, and supersymmetry could be key to uniting all forces into one all-encompassing theory.