A new fundamental force may be very difficult to test. A fifth force could be very feeble, like gravity, causing it to be difficult to detect.
Attila Krasznahorkay
X17 particle
Attila Krasznahorkay, from ATOMKI, the Hungarian institute for nuclear research, in 2015, posited the existence of a new light boson, with the mass of about 17 MeV. This would be about 34 times heavier than the electron. In an effort to find a dark photon, which is a hypothetical particle potentially connected to dark matter, the team targeted thin layers of lithium-7 with protons. This created unstable beryllium-8 nuclei, which then decayed, producing pairs of electrons and positrons. What was observed were excess decays, indicating that some of the beryllium-8 may have been shedding its excess energy in the form of a new particle.
The same anomalies were observed in 2019 in the decay of stable helium atoms as had been observed in the beryllium-8. This strengthened the case for the existence of the X17 particle.