An ancient formation of collapsed protostars whose gravitational wavefronts destroyed the research vessel Vico and nearly claimed the Enterprise-D in the episode Hero Worship.
The Black Cluster formed almost nine billion years ago, when hundreds of protostars collapsed close to one another. The result is a region of violent, shifting gravitational wavefronts that distort sensors and can tear a ship apart.
In the Next Generation episode Hero Worship, set in 2368, the USS Enterprise-D enters the cluster in search of the missing research ship SS Vico. The vessel is found wrecked, with a boy named Timothy as its only survivor. Traumatised, he attaches himself to Data and begins to imitate him.
When the Enterprise meets the same wavefronts, Timothy's memories help Data realise that the ship's own shields are reflecting and amplifying the waves. Dropping the shields at the last moment saves the ship.