The unified field theory was originally a dream held by Albert Einstein believing that all the fundamental forces of nature may be sufficiently unified into a single theory framework. When Einstein attempted to do this, he had an incomplete knowledge of the universe - he was not aware of all the fundamental forces at the time. Even if he had, it is hard to say he would have been able to fare better - such a task is daunting for a number of reasons.
One reason is that such a theory needs to be a mathematically-sound theory which can successfully account for the beginning of our universe and the birth of the forces as we could understand it. Not only is this knowing the mind of God, but the theory also needs to be universally-acceptable so that everyone may come to agree on it - which is a task in itself.
The reason for believing there is such a unification is compelling. Einstein showed that using relativity one can successfully describe magnetism as another side of the electric force. Then of course, Einstein attempted to unify his General theory of Relativity with electromagnetism itself. He largely failed to do this, however Theodor Kaluza a bright German physicist did find a way to unify General Relativity and electromagnetism but at the expense of having to introduce a new dimension of space - the 5th dimension of spacetime to be exact. There is no concrete evidence any more than 3 spatial dimensions exist in nature.
let's be clear what physicists are trying to achieve, they are attempting to describe all the forces in nature under one name; gravito-electro-magneto-strong-weak force. They are wanting to do this by increasing the temperature of the system. The electromagnetic force was successfully unified with the weak force which is responsible for the decay of particles. It was predicted by Sheldon Glashow. In 1967 physicists Abdus Salam and Steven Weinberg had independently revised Glashow's theory by having the masses for the W particle and the Z particle arise through spontaneous symmetry breaking using the Higgs mechanism. Recently we think we have found a Higgs particle however it has around 50% more likelihood to decay into photons than it is predicted by the Standard Model, raising questions whether
a) it is even a Higgs Boson?
b) it is even a Standard Higgs Boson?
Nevertheless, electroweak currents where observed. If one studies the Langrangian (the equations which describe the kinetic energy), electroweak currents have a Langrangian which is split up for a weak part and an electromagnetic part, which indicates that both forces are present at this temperature further indicating they have been unified into different sides of the same the force, just how like electric and magnetic forces are present at low temperatures but are completely relative for moving and stationary systems.
There is a growing number of physicists now who are making the argument that gravity may not not actually be fundamental which of course, presents a large problem in the basic idea of unifying the forces - after all, you can only unify the fundamental forces assuming the force you are dealing with is fundamental! It may not actually be in the same league as the other forces to add to the mystery as it could also be a psuedoforce... in fact Richard Feymann made this point himself at one time. So not only might gravity not be a force in the same sense as electromagnetic or strong or weak nuclear forces... it may not even be fundamental (or better said) it may be emergent.
What does it emerge from?
The answer is that gravity emerges from the geometry of spacetime, alongside matter as well. In fact, John Archibald Wheeler's Geometrogenesis actually explains that geometry is a late phenomenon in the universe, it appears when the universe has sufficiently cooled down. If this is the case, and the source of gravity is in fact the geometry of the universe, then gravity also appeared around the same time and did not originate at the Big Bang under some grand unification like many are led to think. In a sense, it is not hard to understand that maybe we had the idea of this unification wrong to begin with, because so many start theorizing without all the facts.
String theory is a theory which has attempted to unify physics, by introducing an astonishing 11 to 26 new dimensions, depending on which string theory is considered. For many reasons (not all we shall cover in this) physicists don't like this theory very much as it seems to be purely mathematical with little to no experimental data to be predicted from it. Because of that, some have criticized it is ''not even wrong.'' It does on the other hand give a remarkably beautiful picture of gravity, seeping from universe-to-universe connecting all universes like one giant string. In string theory, instead of parallel universes which there can be an infinite amount, string theory deals with a landscape which allows up to 10^(500) vacua.
An unlikely surfer dude by the name of Anthony Garret Lisi was to propose a theory of everything also involving multiple new dimensions. It also predicted new exotic particles outside of the standard model (which, if this newly discovered particle is a Higgs Boson, it is becoming increasingly unlikely there will be any new physics outside of the standard model). His theory is based on the complicated E_(8) Lie Group and it attempts to describe a complicated geometrical web of forces connected to their particles under a unification.
To this day, there is no common agreement on a unified theory and many unified theories often lack a certain appeal. Some of the most daring unified field theories have unfortunately been some of the most complicated and often introducing superfluous amounts of dimensions and new particles. Perhaps within the next 100 years a new Einstein will come along and radically reshape our thinking of the forces at large... on the very small.