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Speculative fifth fundamental force
In physics, there are four observed fundamental interactions (also known as fundamental forces) that form the basis of all known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories. The characteristics of this fifth force depend on the hypothesis being advanced. Many postulate a force roughly the strength of gravity (i. e., it is much weaker than electromagnetism or the nuclear forces) with a range of anywhere from less than a millimeter to cosmological scales. Another proposal is a new weak force mediated by W′ and Z′ bosons. The search for a fifth force has increased in recent decades due to two discoveries in cosmology which are not explained by current theories. It has been discovered that most of the mass of the universe is accounted for by an unknown form of matter called dark matter. Most physicists believe that dark matter consists of undiscovered subatomic particles that are difficult to detect, but some believe that it could be related to an unknown fundamental force. Second, it has also recently been discovered that the expansion of the universe is accelerating, which has been attributed to a form of energy called dark energy. Some physicists speculate that a form of dark energy called quintessence could be a fifth force.

Theoretical proposals in the fifth force category are driven by two inconsistencies between the current models of general relativity and quantum field theory, the hierarchy problem and the cosmological constant problem. Both issues suggest the possibility of corrections to the gravitational potential around and very long baseline interferometry.

Another kind of fifth force, which arises in Kaluza–Klein theory, where the universe has extra dimensions, or in supergravity or string theory is the Yukawa force, which is transmitted by a light scalar field (i. e. a scalar field with a long Compton wavelength, which determines the range). This has prompted a much recent interest, as a theory of supersymmetric large extra dimensions with size slightly less than a has prompted an experimental effort to test gravity on very small scales. This requires extremely sensitive experiments which search for a deviation from the inverse square law of gravity over a range of distances. Essentially, they are looking for signs that the Yukawa interaction is engaging at a certain length. Australian researchers, attempting to measure the gravitational constant deep in a mine shaft, found a discrepancy between the predicted and measured value, with the measured value being two percent too small. They concluded that the results may be explained by a repulsive fifth force with a range from a few centimetres to a kilometre. Similar experiments have been carried out on board a submarine, USS Dolphin (AGSS 555), while deeply submerged. A further experiment measuring the gravitational constant in a deep borehole in the Greenland ice sheet found discrepancies of a few percent, but it was not possible to eliminate a geological source for the observed signal.

Another experiment uses the Earth's mantle as a giant particle detector, focusing on geoelectrons.

Jain et al. (2012) examined existing data on the rate of pulsation of over a thousand cepheid variable stars in 25 galaxies. Theory suggests that the rate of cepheid pulsation in galaxies screened from a hypothetical fifth force by neighbouring clusters, would follow a different pattern from cepheids that are not screened. They were unable to find any variation from Einstein's theory of gravity.

Some experiments used a lake plus a tower that is 320eters high. A comprehensive review by Ephraim Fischbach and Carrick Talmadge suggested there is no compelling evidence for the fifth force, though scientists still search for it. The Fischbach–Talmadge article was written in 1992, and since then, other evidence has come to light that may indicate a fifth force. The above experiments search for a fifth force that is, like gravity, independent of the composition of an object, so all objects experience the force in proportion to their masses. Forces that depend on the composition of an object can be very sensitively tested by torsion balance experiments of a type invented by Loránd Eötvös. Such forces may depend, for example, on the ratio of protons to neutrons in an atomic nucleus, nuclear spin, or the relative amount of different kinds of binding energy in a nucleus (see the semi empirical mass formula). Searches have been done from very short ranges, to municipal scales, to the scale of the Earth, the Sun, and dark matter at the center of the galaxy.

In 2015, Attila Krasznahorkay at ATOMKI, the Hungarian Academy of Sciences's Institute for Nuclear Research in Debrecen, Hungary, and his colleagues posited the existence of a new, light boson only 34 times heavier than the electron (17 MeV). In an effort to find a dark photon, the Hungarian team fired protons at thin targets of lithium 7, which created unstable beryllium 8 nuclei that then decayed and ejected pairs of electrons and positrons. Excess decays were observed at an opening angle of 140° between the e and e, and a combined energy of 17 MeV, which indicated that a small fraction of beryllium 8 will shed excess energy in the form of a new particle. In November 2019, Krasznahorkay announced that he and his team at ATOMKI had successfully observed the same anomalies in the decay of stable helium atoms as had been observed in beryllium 8, strengthening the case for the X17 particle's existence. Feng et al. (2016) The force may explain the muon anomaly and provide a dark matter candidate. Several research experiments are underway to attempt to validate or refute these results.