Quantum field theory is a higher level quantum theory, based on the postulates of quantum mechanics. . The basic quantum mechanical wave equations of Schrodinger, Dirac, Klein Gordon, quantized Maxwell, are solved for two particles in a potential. In order to model the observed scattering and decays of elementary particles field theory was developed in the following way:
The plane wave solutions of the above equations are assigned as fields in all points of space time, electron fields,quark fields,neutrino fields and their antiparticle fields, a mathematical coordinate system, like a Lorenz invariant aether. On these plane wave solutions/fields differential operators operate creating and annihillating the named particles. This allow for the Feynman diagrams that allow to calculate two body to many body scatterings and decays of particles in a strict mathematical way. This model has been very successful in modelling and predicting data in particle physics, nuclear physics as well as superconductivity etc.
So the fields are tied up irrevocably to the operators, it is the system of both that creates and annihilates particles.
or is it possible that by some way, all the particles/matter in the universe gets converted back in to their respective fields
Fields cannot be discussed without the operators. It has no physical meaning to just discuss fields . They are a mathematical construct. It is the combination of fields and operators that create the observable states.
Just as things were instantaneously after the big bang - no particles and only fields ?
This is not correct.When quantum mechanics became part of the Big Bang model, the inflaton field and particle had to be invented in order to describe the beginning of the universe quantum mechanically.
You can only have a quantum field theory with fields coupled to operators that create particles. That is why it was very important to find the Higgs particle in order to validate the standard model.