Is the mass of a particle taken as an exact number by empirical experimentation? In this question the first comment says that it would be easy to test for variation in mass of particles, but none has been seen; so I wonder: Is the mass of a particle an exact number by experience? Or is it a consequence of some theory in physics?
Thanks in advance.
 A: 
Is the mass of a particle an exact number by experience? Or is it a consequence of some theory in physics?

The masses of particles and resonances are meticulously  measured over the last hundred years and the values of the mass and quantum numbers determined experimentally are recorded in the Particle Data Group summaries.
The mainstream theoretical model , modeling the interactions of these particles, is the standard model, which uses quantum field theory for its calculations. Theory is continually developing , no one has claimed that there exists a Theory of Everything at present, so the existence of the particles cannot be a consequence of a theory. Particles exist, theories model their interactions.
A: It is certainly interesting that every electron (any particle but I'm using an electron as an example) has the exact same mass and spin and charge no matter how we look at it or where and when we measure it.  This has been the foundation of many physical theories.  Most notable to me would be the one electron universe which states that may be all electrons are the same exact electron going forward and backwards in time.
This later inspired Fyenman who changed how we now understand it that every electron is a an excitation of the exact same underdlying quantum field that permeates everywhere.
