We started to have a completely new view about the creation, existence and evolution of the Universe after Albert Einstein published his General Theory of Relativity which predicted the unification of four fundamental forces, which are sustainer and maintainer of this ever changing Cosmos of Energy and Matters. 
Scientists have theories and mechanism to prove the presence of Strong and Weak forces as it can be felt in very small scale mediums like Strong force clustered together all the necessary quarks to form a proton, electron and neutron like particles which are most fundamental fraction of an Atom and again at large scale it holds together Proton and Neutron to maintain Nucleus of Atoms.
While Weak force is one of the strongest forces, known as Weak force due to its short range of functioning. The weak force is responsible for the creation and transformation of elementary particles and plays its part in decaying of elements which caused them to transform into another element. It’s most efficient and common process is the transformation of Neutron into Electron and Proton. W and Z bosons take part in exchanges which cause Weak interaction. 
But Gravity and Electromagnetic forces play their parts. Electromagnetic force is responsible of the formation of molecules by binding different type of Atoms together and Gravity is responsible for binding and maintains all the matters and energy that is available in the Universe.
Since conception of gravity we could not fully understand it as gravity works at very large scale. Albert Einstein had spent all his life to understand gravity but he was not very much sure about exact mechanism.
Some scientists had even hypothesized that a particle name Graviton exists which takes part in exchange to create a gravitational field. But till today Graviton has not been discovered.
As by the standards of classical physics, gravity is due to the matter. Magnitude of Gravity between two bodies depends on their masses and distance, but once studies further it has come into attention that there is more gravity in the Universe than the total available mass.
Some scientists have speculated that some hidden entity may exist which cannot be seen they named it ‘ Dark Matter’ but no one has been able to explain it perfectly neither there is any evidence to prove it.
To explain the things in a better way some scientists have proposed that a fifth force may exist which could explain all out unsolved mysteries. Researchers have tried to monitor the presence of fifth force in our Solar System but the magnitude of gravitational field is so low here that they could not figure out anything from those data.
Then a new method was proposed to monitor the behavior of stars near the center of our Galaxy Milky Way, where the gravitational field is very strong due to the super massive black holes. They proposed to monitor the path and movement of the stars near the center of the galaxy where it will be easy to detect any other force than gravity which influences the movement of stars.
Scientists have planned to monitor these movements of stars for more than 20 years to devise a perfect conclusion. Two decades are a long time who knows before that we may have some other theories to explain these unsolved mysteries. Till they don’t find any evidence to prove, fifth force is just a hypothesis.