In physics, the fifth force is a proposed fundamental force, additional to the four known fundamental forces of nature. The conventionally accepted fundamental forces that form the basis of all known interactions are the gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories; the characteristics of this fifth force depend on the theory being advanced. Many postulate a force roughly the strength of gravity(i.e. it is much weaker than electromagnetism or the nuclear forces) with a range of anywhere from less than a millimeter to cosmological scales. Another proposal is a new weak force, mediated by W’ and Z’ bosons.

The search for a fifth force has increased in recent decades due to the discovery that most of the mass of the universe is accounted for by an unknown form of matter called dark matter. Most physicists believe that dark matter is some new undiscovered subatomic particle, but some believe that it could be related to an unknown fundamental force. It has also recently been discovered that the expansion of the universe is accelerating, which has been attributed to a form of energy called dark energy, and some physicists speculate that a form of dark energy called quintessence could be the fifth force.^[1][2][3

https://en.wikipedia.org/wiki/Fifth_force