  Diffuse gamma-ray emission from the decay of radioactive $^{26}$Al is a
messenger from the nucleosynthesis activity in our current-day galaxy. Because
this material is attributed to ejections from massive stars and their
supernovae, the gamma-ray signal includes information about nucleosynthesis in
massive star interiors as it varies with evolutionary stages, and about their
feedback on the surrounding interstellar medium. Our method of population
synthesis of massive-star groups has been refined as a diagnostic tool for this
purpose. It allows to build a bottom-up prediction of the diffuse gamma-ray sky
when known massive star group distributions and theoretical models of stellar
evolution and core-collapse supernova explosions are employed. We find general
consistency of an origin in such massive-star groups, in particular we also
find support for the clumpy distribution of such source regions across the
Galaxy, and characteristics of large cavities around these. A discrepancy in
the integrated $^{26}$Al gamma-ray flux is interpreted as an indication for
excess $^{26}$Al emission from nearby, distributed in cavities that extend over
major regions of the sky.
