NGC-21997 was a black hole located in the vicinity of the Mariah, Dreon, and Volnar systems in the Alpha Quadrant, near Cardassian and Ferengi space. It was 37 solar masses in size.
In 2271, Captain Krell of the Klingon Empire attacked the Bajoran Militia starship RBS Verda near NGC-21997. Colonel Shad Yima, CO of Verda, fled into the black hole's gravity well to hide from pursuit in the interference, but both ships suffered damage to their warp drives, causing time dilation to displace them into the early 25th century.
In 2410, USS Bajor was passing NGC-21997 when the Astrometrics department detected a severely redshifted distress signal from Verda. The Federation ship traveled to the black hole and beamed the crew off, later returning to tow Krell's D-7 to safety. (Bait and Switch: "A Changed World")