In a private email correspondence Prof Sabbagh confirmed to me that the definition of galaxy that I guessed is correct: as strong inaccessible cardinals are to universes, strong limit cardinals are to galaxies. The intent of the thesis was to examine a desideratum for ZF-Foundation involving the closure of universes and galaxies under Hilbert $\tau$ operator. This turned out to imply the axiom of Foundation, which was the main result of the thesis, and published in

• Gabriel Sabbagh Les univers et l'axiome de fondation, Arch. Math 20 (1969) 449—452, https://doi.org/10.1007/BF01899449