Even if the mass center of the universe cannot be located in reality, it has been shown to exist. We can now use the concept in the development of a reasonable philosophy of science to replace the relativity principle which has plagued humanity so long. The concept of the center of mass is purely mathematical. In the case of a distributed mass, which applies for any object having a volume, the mathematical analysis is greatly simplified if we can assume that the entire mass is located at a single point, and a single force can be applied to cause motion. This can only be true in case there are internal forces in the object of such a nature as to establish a coherent structure.
Particle theory is relatively new.
The kinetic theory of gases gave no insight into the nature of the molecule although it recognized its existence. The electron was discovered to exist only a hundred years ago. Einstein considered particles of matter as space singularities. This name was applied only in the sense of recognizing the fact the known laws of physics did not apply at such a point. If we consider that the only known law of force was that of the inverse square, the force became infinite as the zero value of the radius vector was approached.