There is symmetry in the nature. Then, there should also be symmetry in physics since physics describes the phenomena of nature. In fact, it occurs in most of the phenomena explained by physics as for example: a particle has positive or negative charges, spins up or down, north or south magnetic poles. In this form, the particle should also have mass symmetry. For convenience and due to later explanations, I call this mass symmetry or mass duality as follows: mass and mass cloud. The mass symmetry can be corroborated in the experiments of the hydrogen spectrum, the Bohr model and the solution of the Schrödinger equation. The mass cloud is located in the respective orbitals given by the Schrödinger equation. The orbitals represent the possible locations or places of the particle which is determined probabilistically by the respective Schröndiger equation. For the proton, part of the mass of the uncharged proton is distributed in the orbital or mass cloud around the mass that contains the positive charge. Thus, the positive charge in the proton is concentrated in its mass nucleus with an uncharged mass cloud around its nucleus distributed in the orbitals. For the electron, part of the mass of the uncharged electron is distributed in the orbital or mass cloud around the mass that contains the negative charge. Thus, the negative charge in the electron is concentrated in its mass nucleus with an uncharged mass cloud around its nucleus distributed in the orbitals. For example, in the formation of the hydrogen atom, a part of the mass cloud of the proton interacts with the mass cloud of the electron, and the total mass energy lost in this interaction is transformed into electromagnetic energy according to Einstein's equation: E=mc2 and the variant mass formula discovered and developed by myself. Then, the two particles join together due to this interaction and the electrostatic force between the two particles. Therefore, the electron and proton are bound together in the hydrogen atom by the mass cloud of the electron and proton with some mass cloud lost in the interaction and converted to electromagnetic energy or photons. Then, it is right this mass symmetry, since the electron and the proton in the interaction of the mass cloud lose mass but do not lose electric charge. In this form, it is justified the existence of a mass cloud. In the formation of the Hydrogen atom, the electron-proton system when approaching gains a potential energy of 27.2 eV (13.6 eV*2) but then when the electron bond occurs in the shell with quantum state n =1, energy of 13.6 eV is emitted as electromagnetic energy or photons and the remaining 13.6 eV remains as kinetic energy of the electron. Then, the Hydrogen atom has 13.6 eV of additional energy/mass than the sum of the energy/mass of the proton plus the electron. Therefore, 13.6 eV is needed to ionize the Hydrogen atom and expel the electron from the atom. The mass/energy reduction of the proton and electron is 13.6/2 eV for each particle due the emission of 13.6 eV as electromagnetic energy. Therefore, the main function of the mass cloud is the binding energy. The mass cloud interaction generates binding energy between the electrons and the nucleus in the atom through the protons and between the nucleons in the nucleus: protons with protons, neutrons with neutrons, and protons with neutrons. The nuclear force between two nucleons is characterized by being strong and short-range. Also, it can be justified by the existence of the mass cloud: the mass clouds of nucleons within the nucleus interact with each other without any effect on the proton charge. This scientific research presents evidence of the existence of the mass symmetry based in the Einstein's equation and in the Variant Mass formula for the Electron in the atom discovered and demonstrated by myself where experimental results are detailed.