"fifth force." YourDictionary, n.d. Web. 14 January 2019. <https://www.yourdictionary.com/fifth-force>.
fifth force. (n.d.). Retrieved January 14th, 2019, from https://www.yourdictionary.com/fifth-force
Any of various hypothetical, very weak forces thought to cause bodies to repel each other. Such forces are occasionally considered as a solution to unexpected deviations in the measured value of the gravitational constant or to explain the apparent acceleration of very distant galaxies away from earth.