Pin galaxies were galaxies which existed within the universe of the atom. They were difficult to study as they only had a life span of about one atto-second (a quintillionth of a second). Dastari's people were researching rho mesons as the unstable factor in these galaxies. (TV: The Two Doctors)
One reported to the Hermit that some remains of Mictlan technology were leading out into pin galaxies with variant physical laws, but it was nothing that might attract Swimmers. (PROSE: The Taking of Planet 5)