A black cluster

A map of a black cluster

A black cluster was a type of cluster that was created when numerous protostars collapsed in close proximity to one another. The result of this collective collapse was an area of space that absorbed energy and dangerously affected spacecraft systems. Prior to 2368, the Federation maintained reports from starships traveling through black clusters indicating sensor difficulties.

In 2368, the SS Vico was assigned to explore a black cluster, located in Sector 97. This particular cluster was formed about nine billion years ago, from hundreds of collapsed protostars, and was seven times more massive than any black cluster that had been previously sighted. After Starbase 514 lost contact with the Vico, the USS Enterprise-D was sent to investigate. Upon its arrival on the edge of this black cluster, the destroyed hulk of the Vico was immediately found. The Enterprise itself was almost destroyed while further investigating the Vico's disappearance. (TNG: "Hero Worship")

In ultimately omitted dialogue from the fourth draft script of "Hero Worship", the gravitational forces in the creation of that episode's black cluster were said to have been so strong that "the object went even beyond the black hole stage," resulting in the black cluster, which was then described as "essentially an anti-gravitational object."
In the stage directions from the final draft of the "Hero Worship" script, the black cluster was initially characterized as "a huge, black bending of space, seemingly an immense lens-like cloud, dotted throughout by tiny pulsars." [1]
According to Star Trek: Star Charts ("United Federation of Planets I") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the black cluster explored by the Vico was located in the Alpha Quadrant.
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