Electromagnetism, gravity, strong nuclear force and weak nuclear force are the four known fundamental forces.
While doing experiments in searching for “dark photons,” particles that would signify unseen dark matter, they uncovered an anomaly that hinted at the existence of a light particle that was 30 times heavier than an electron.
“The experimentalists weren’t able to claim that it was a new force. “They simply saw an excess of events that indicated a new particle, but it was not clear to them whether it was a matter particle or a force-carrying particle.”
If confirmed by further experiments, this discovery of a possible fifth force would completely change our understanding of the universe and Standard Model of physics could be due a major revision.
Source :
http://www.cnet.com/news/science-may-have-found-a-fifth-force-of-nature-to-tie-together-the-loose-ends-of-the-universe/
https://en.wikipedia.org/wiki/Fifth_force