Black Hole
A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. Around a black hole, there is a mathematically defined surface called an event horizon that marks the point of no return.
The nearest black hole to Sol is V4641 Sagittarii and this is 7,350 parsecs away.
No black holes have ever been visited, but calculation suggest that they would typically have an M25 limit of several parsecs - making close observation impossible.
During the ESS Odyssey mission careful attention was paid to the potential hazards of encountering a black hole's spacetime incursion into hyperspace as a mission critical navigational hazard, and the course of this explorer ship carefully plotted accordingly.