A hole in space was the description given to the phenomenon created by Nagilum in the Morgana Quadrant in order to entrap the USS Enterprise-D in 2365. Unlike normal space, the phenomenon was totally devoid of any objects. Having no discernable matter or energy yet had a visible volume. Having rendered the Enterprise immobile. The sight of the void reminded Worf of an ancient Klingon legend about a "giant black space creature" that was said to devour entire vessels.
Nagilum created the hole in order to study the Humans aboard Enterprise. To this end it also created an image of the USS Yamato to study the Humans' reaction and killed a member of the Enterprise crew in order to understand their reaction to death.
Nagilum eventually released the starship when it became clear that Captain Jean-Luc Picard and the rest of the crew would prefer to destroy the ship by auto-destruct rather than remain as prisoners of the creature. (TNG: "Where Silence Has Lease")