Now, as new research has shown, a fifth force could also be connected to dark matter. In a paper published in Physical Review Letters, physicists Jo Bovy and Glennys Farrar were surprised to discover that a fifth force in the dark sector could place constraints on dark matter that essentially exclude its direct detection through spin-independent interactions. Conversely, if future experiments do detect a spin-independent interaction of dark matter, then any fifth force in the dark sector must be so weak as to be astrophysically irrelevant.

"Our study shows that we can strongly constrain some properties of dark matter, i.e., the combination of its interaction with the visible sector and the strength of a long-range fifth force between dark matter particles, through experiments with ordinary matter," Bovy, a Ph.D. student at New York University, told PhysOrg.com. As for which scenario appears to be more likely - a fifth force excluding direct detection of dark matter, or direct detection of dark matter excluding a relevant fifth force - Bovy and Farrar said that it's impossible to say in advance. "Both would be very interesting both theoretically as well as observationally," Bovy said.

Previous research has suggested the possibility that a new long-range, attractive fifth force might exist, which arises in several extensions of the standard model. Although most dark matter models predict that the force between dark matter particles is a short-range force, other models such as supersymmetry and string theory allow for the existence of a very light boson which could carry a long-range force in the dark sector.