The Mar Oscura Nebula was a dark matter nebula that was located in a sector of the Alpha or Beta Quadrant. It contained several class M planets.
In 2367, the USS Enterprise-D became the first manned Federation starship to explore this nebula.
A deformation of space as seen on a shuttlepod display graphic
Using a series of specially modified photon torpedoes, the Enterprise crew was able to illuminate the nebula. Studying the scans taken of the Mar Oscura, Lieutenant Commander Data found that the dark matter density was nearly one order of magnitude higher than in similar nebulae. It was soon discovered that, due to the unusual preponderance of dark matter, small gaps in the fabric of normal space had formed, causing part of the ship to momentarily phase out of normal space each time the Enterprise collided with one of these deformations. This made navigation in the nebula extremely hazardous and even resulted in the death of a crewmember, Lieutenant junior grade Van Mayter. In order to safely exit the nebula, Captain Picard piloted a shuttlecraft ahead of the Enterprise-D, so he could detect the deformations ahead of time and relay the course corrections to the ship. As Picard was piloting the shuttle, a deformation collided with the shuttle's starboard impulse nacelle, necessitating an emergency transport of Picard back to the ship moments before the shuttle was destroyed. With only one million kilometers remaining to the outer perimeter, Commander Riker ordered the Enterprise out of the nebula. (TNG: "In Theory")