There have been news reports about the possible discovery of a fifth force of nature. The source of the interest is a paper titled Protophobic Fifth-Force Interpretation of the Observed Anomaly in 8Be Nuclear Transitions that appears in the August 12, 2016 edition of Physical Review Letters.
Here is the introduction to the paper.
The four known forces of nature, the electromagnetic, weak, strong, and gravitational inter- actions, are mediated by the photon, the W and Z bosons, the gluon, and the graviton, respectively. The possibility of a fifth force, similarly mediated by an as-yet-unknown gauge boson, has been discussed since shortly after the introduction of Yang-Mills gauge theories, and has a rich, if checkered, history. If such a force exists, it must either be weak, or short ranged, or both to be consistent with the wealth of experimental data. In recent years, interest in this possibility has been heightened by the obvious need for dark matter, which has motivated new particles and forces in a dark or hidden sector that may mix with the visible sector and naturally induce a weak fifth force between the known particles.
Recently, studies of decays of an excited state of 8Be to its ground state have found a 6.8σ anomaly in the opening angle and invariant mass distribution of e+e− pairs produced in these transitions. The discrepancy from expectations may be explained by as-yet-unidentified nuclear reactions or experimental effects, but the observed distribution is beautifully fit by assuming the production of a new boson. In this work, we advance the new particle interpretation, carefully considering the putative signal and the many competing constraints on its properties, and present a viable proposal for the new boson and the fifth force it induces.
By bombarding 7Li nuclei with protons of particular energies the researchers are able to create two excited states of 8Be and then observe their decay. What they notice are bumps in the spectra that they claim cannot be explained by nearby resonances but can be explained by postulating that a new vector gauge boson particle X with mass 16.7 MeV is produced that promptly decays into an electron-positron pair. They argue that this particle mediates a new fifth force with a range of 12 fm.
The prediction of new fifth forces has happened in the past to explain anomalies and then subsequently faded away, so as with all such predictions, it is best to wait and see what subsequent investigations turn up before jumping to the conclusion that a new particle and associated force actually exists.