This may probably seem an unusual question in physics however, taking it into consideration may lead us to solve some of the problems in this science.
As every physicist knows, in QM and Relativity, it has been accepted that field and mass-energy are two separable items.
In G.R., gravity is replaced by space-time, therefore it is not a fundamental force. Q.M. is a very good set of mathematical models that show how many elementary forces work, but it does not explain how they work. What is the main obstacle in the way of uniting the four forces and all of the elementary particles? We do not know how a charged particle produces an electric field or virtual photons in Q M. And many other unanswered questions. Maybe thinking about this seems useless or maybe it can be a step in order to find a theory of supersymmetry.