From extensive studies in the optical range we know that Early-Type Galaxies, which host most of the stellar mass in the Universe, formed the vast majority of their stars at z>2 in rather short timescales (<1 Gyr). These results are in tension with semi-analytic models of hierarchical structure formation and latest generation of cosmological hydro-dynamical simulations, which predict significantly higher levels of young and intermediate age stellar populations. As we are aware that the optical range is biased towards red, old, stars we recently extended our stellar population synthesis models to predict for the first time spectra at moderately high resolution in the UV based on empirical stellar templates observed with the HST. The application of these models to stacked spectra of thousands of massive ETGs in both the optical and the UV ranges have allowed us to detect sub one percent mass fractions of stellar populations with ages younger than 2 Gyr. Moreover we find that these fractions decrease slightly with increasing galaxy mass, consistent with a downsizing scenario. In addition, deep spectra of nearby massive galaxies have allowed us to determine their gradients. We conclude that these young contributions are mostly concentrated in their innermost regions and become virtually negligible at the effective radius. Such young contributions are consistent with residual star formation from gas returned to the interstellar medium from late phases of stellar evolution, although ex-situ contributions that reach central galaxy regions triggering the star formation is not excluded. These results pose unprecedented, stringent, constrains to galaxy formation models.