You guys n gals are going to love this one
http://www.sciencealert.com/physicists-think-they-might-have-just-detected-a-fifth-force-of-nature?
EDIT:http://phys.org/news/2016-05-case-nature.html
So two teams now say they are observing a spike of something between electron and protons mass emitted when protons are bombarded into an isotope of litium.
Of course to support the claim Eureka Alert pointed to a wiki page on it.
https://en.m.wikipedia.org/wiki/Fifth_force
Of course if you think that there is a consistent thought on the matter think again
Some think this is a:
dark photon or some form of dark matter, photons of course don't have rest mass so they are proposing some kind of massive dark photon, yeah I said it, don't shoot the messanger.
Something about as strong as gravity, here again not sure what the evidence is. These are based on a variance of the gravitational constant by very small values when an object is surounded by increasing amounts of mass, problem is that the constant also varies with temperature and other varieties of change. The constant itself may only be conditionally constant.
Something that explains dark energy and dark matter, and gravitational interactions at great distances.