For every particle of matter, there is a partner particle that is a carrier of force, and for every particle that is a carrier of force, there is a corresponding matter particle. Examples of matter particles are electrons and quarks, while an example of force-carriers is the photon, whose "exchange" between charged particles mediates the electromagnetic force.

This new symmetry between particles of matter and carriers-of-force is called supersymmetry.

Among the super-partner particles, most decay into lighter particles, including at least one super-particle. However, there must be one super-partner that is the lightest, called the LSP (lightest supersymmetric particle). The LSP is stable (cannot decay) and is a permanent component of our universe. As a result it is a candidate for the so-called dark matter, the approximately 80% of the total mass of the universe that is not visible to astronomers as luminous matter.