Mar Oscura Nebula

From Memory Alpha, the free Star Trek reference.

The Mar Oscura Nebula is a dark matter nebula which remained unexplored until 2367. Although containing dark matter pockets and several Class M planets, it is also littered with spatial deformations which makes travel dangerous. The USS Enterprise-D discovered these deformations when it charted the area on stardate 44932.3. (TNG: "In Theory")