Here, applying the lacking part of ultimate theory i.e. the Scale-Symmetric Theory, we proved that the unidentified emission line approximately 3.56 keV follows from the weak and electromagnetic interactions in plasma composed of fully ionized atoms of iron (the atomic nuclei of iron) and of free electrons. Temperature of the plasma is adequate for the captures of the free electrons by the ions in their n = 1 state, where n is the principal quantum number. Next, there are the transitions of the emitted X-rays/photons into electromagnetic-type or weak-type weak condensates inside protons. The energy of the weak condensates leaks from protons and again transforms into electromagnetic energy. Such two succeeding transitions via the electromagnetic-type weak condensate decrease the initial energy to 3.5566 keV - it is some analogy to the phenomenon dominating in a LASER, i.e. the wave amplification by stimulated emission of radiation. The obtained theoretical result (3.5566 keV) is consistent with the MOS and PN (they are the two different CCD detectors on XMM-Newton) observations. Probability of creation of the weak-type weak condensates is very low - their production decreases the initial energy to 3.5157 keV.