I heard about in high energy the 4 fundamental forces can combine. They were not separated until $10^{-11}$ seconds after the big bang and as the univers gets cold, they begun to seperate.
We generally regard a force as fundamental if it is mediated by a fundamental particle. For example the EM force is mediated by photons, the strong force by gluons and the weak force by $W$ and $Z$ particles. We don't have a quantum theory of the gravitational force, but we generally include it in the fundamental forces because we have no reason not to (though there are regular suggestions that gravity is an emergent force and not fundamental).
As it happens there is a fifth force mediated by the Higgs boson, though describing this as a fundamental force is arguably a bit pedantic.
Anyhow, extra forces would require new fundamental particles to mediate them. As of right now we have no evidence for any particles beyond the ones described by the standard model, so as of right now there is no evidence for a fifth fundamental force. However there is also no evidence that extra particles cannot exist so there is no evidence that extra forces cannot exist.
The scientific media is replete with suggestions for fifth forces - one just recently - and Googling will find no end of candidates. However all the apparent discoveries of a fifth force have proven groundless so far.
The idea of a $5^{th}$ force has been around for a while. E. Fishbach made a big claim about this in the 1980s, which never worked out. As John Rennie points out we could consider the Higgs field a fifth force. In fact the quartic potential of the Higgs field gives a Lagrangian term similar to the $(A\wedge A)^2$ term in the Lagrangian for a Yang-Mills gauge field. However, the field is a spin-0 scalar field and not a vector or chiral field with spin-1.
The quest for a fifth force, or maybe some set of additional forces, has some basis. First there does not exist a “no-go theorem” that says there can't exist such a force. The other is that with dark matter it may be the case there is some sector of physics with fermions and gauge bosons that are entirely different from those we understand. Gravitation is universal and so this will interact by gravity due to the equivalence principle(s). The equivalence principle of gravitation may be due to the universal nature of quantum entanglements. If so there should be some “hook” that connects these sector of physics to our own. The gauge bosons and maybe their fermions may interact with matter we understand as a result.
This idea of dark matter is in contrast to the supersymmetric phenomenology that ties dark matter to super-partners of known particles. The standard such model is the neutralino, which is a condensate of the photonino, Zino, and Higgsino. This form of dark matter should interact by the weak interaction, and are the WIMPS. However, the search for WIMPS has been null and the LHC search for low mass supersymmetric particles has come up empty so far. With the LUX-Zeplin that will be up in a few years the search for DM will continue. However, supersymmetric phenomenology at GeV-TeV energy appears in danger of being demolished.
A paper has recently appeared that interprets an anomalous nuclear transition according to a gauge boson that is different from known forces. This gauge boson fills a $17MeV$ energy gap as a presumed gauge boson of that mass. This occurs from an internal pair production in $^8Be$. Beryllium has two excited states above a ground state of $18.14MeV$ and $17.64MeV$. These excited states all have spin-1 and transition to the ground state of spin-0. This orbital angular momentum in a p-wave produces a photon with $s~=~1$. This process can occurs with the production of an $e-e^+$ pair from a virtual photon in the nucleus. This process due to an off-shell photon has an anomalous angle between them. The $dN/d\theta$ should decline wtih respect to $\theta$. This anomaly can be counted for with an additional massive spin = 1 particle or an unknown gauge boson. However there is a bump in this data at $\theta~\in~(120^o.~140^o)$ the authors cites to within $6.5$ sigma. So these data are not easily dismissed right off. This process is called proto-phobic, for it does not produce a proton so the nucleus decays into $^7Li$.
At this stage this is something to keep an eye on. I would not say the case for a fifth gauge boson is completely settled. These experiments need to be expanded to see if there are other such nuclear processes with different nucleons. It is entirely possible there is a fifth force here. It could also be the case that something is not right with our understanding of the nuclear physics. It is the case that many great discoveries come not with a shout of "eureka," but with a muttering of "well that's funny." So nature could have surprises up her sleeves.
