A rogue solar system in interstellar space might fit the description (kind of like a rogue planet). For example gravitational interactions in the past launched the solar system outside of its parent galaxy, and now it is just floating in space faraway from any light, or galaxy. Your species reached scientific maturity after the solar system already disappeared into interstellar space.
In this case the solar system might seem like all that exists, since everything else would not be visible. However in this case the solar system is functionally isolated, and is basically the only "universe" they need to care about.