"fifth force." YourDictionary, n.d. Web. 12 December 2018. <https://www.yourdictionary.com/fifth-force>.
fifth force. (n.d.). Retrieved December 12th, 2018, 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.