The Stilph were a warp-capable species who inhabited a region just inside the galactic barrier. Their existence was documented by the Sphere.
As of 3190, the Stilph had never encountered the Federation, though they had dealt with the Orions. That year, Captain Michael Burnham approached Haz Mazaro about obtaining Stilph stellar surveys that could contain data about the extragalactic origin point of the Dark Matter Anomaly. These surveys revealed the presence of a massive hyperfield created by Unknown Species 10-C. (DIS: "All In")
This species was only mentioned in dialogue.