Supersymmetry
Many physicists have developed theories of supersymmetry, particularly in the context of Grand Unified Theories, which
attempt to unify the Strong, Weak, and Electromagnetic interactions to help explain why particles have the masses they have. The supersymmetric
theory postulates that every particle we observe has a massive
"shadow" particle partner. For example, for every quark there may be a
so-called "squark" tagging along.
No supersymmetric particle has yet been seen, but experiments
are underway at CERN and
Fermilab, two of the biggest particle physics research facilities in the world.
One of the supersymmetric particles (the "neutralino") might make up the missing dark matter.