Probably the single most mysterious alien civilization ever encountered, humanity has yet to learn if the Sphere Builders are one species or many. They may even be extinct, automated defenses of vast complexity keeping others at bay.
What mankind does know is that literally millions of stars within the galaxy have been enclosed entirely via artificial means, exactly as speculated about once and referred to as “Dyson Spheres.” They run the gamut in size, although many more (by almost an order of magnitude) seem to encase dwarf stars such as Sol or Proxima Centauri than around giants (mostly likely red giants that have a vast lifespan).
These spheres seem totally resistant to analysis, and vessels or probes that get within several AUs of the spheres simply shatter or break apart. Pieces recovered seem to indicated the machines were crisscrossed simultaneously with hundreds or thousands of strands of microscopic material that leaves no trace of any kind, yet rips through diamond as if it were water.
No one else claims to know anything about about the Sphere Builders either, save that these spheres seem to have been in place for millions of years. Legends speak of stars suddenly dimming, and investigation revealing a new sphere, but this has evidently not happened in living memory. Maybe some ancient races know more than they are saying, but no member of the human race seems to know anything about it. Whether the Sphere Builders are even in communication with each other remains unknown although if they are it must be via some means undetectable by anyone willing to talk about it.
It should surprise no one that some groups have assigned a religious value to the Spheres, which includes habitats and ships and sometimes even rogue planets now in orbit around some of the Spheres viewed as places of pilgrimage and meditation, sometimes veneration and sacrifice.