  HII regions, ionized nebulae where massive star formation has taken place,
exhibit a wealth of emission lines that are the fundamental basis for
estimating the chemical composition of the Universe. For more than 80 years, a
discrepancy of at least a factor of two between heavy-element abundances
derived with collisional excited lines (CELs) and the weaker recombination
lines (RLs) has thrown our absolute abundance determinations into doubt. Heavy
elements regulate the cooling of the interstellar gas, being essential to the
understanding of several phenomena such as nucleosynthesis, star formation and
chemical evolution. In this work, we use the best available deep optical
spectra of ionized nebulae to analyze the cause of this abundance discrepancy
problem. We find for the first time general observational evidence in favor of
the temperature inhomogeneities within the gas, quantified by t2. The
temperature inhomogeneities inside H II regions are affecting only the gas of
high ionization degree and producing the abundance discrepancy problem. This
work implies that the metallicity determinations based on CELs must be revised,
as they can be severely underestimated, especially in the regions of lower
metallicity, such as the JWST high-z galaxies. We present methods to estimate
these corrections, which will be critical for robust interpretations of the
chemical composition of the Universe over cosmic time.
