Dark matter

 was a type of matter which was believed to comprise most of the universe's total mass. It could not be perceived directly, and its existence was inferred from its gravitational effects on visible matter.

The Millennium War began when the Mad Mind of Bophemeral attempted to use a dark matter cloud in the Rift of Perseus but was stopped by the Dæmons. (PROSE: The Quantum Archangel)

The Seventh Doctor and Benny used the Jade Pagoda to escape a quasi-baryonic dark matter black star. (PROSE: Sanctuary)

The Spire was built by the Vo'lach to get matter from many alternate ends of the Universe. This was used to create tachyonic dark matter (unintentionally making quasars), which prolonged the existence of the universe in which the Spire existed. (PROSE: Ghost Devices)

The Silver Devastation was an area of space in the Isop Galaxy that contained nothing but dead stars and dark matter. (PROSE: The Game of Death)

Jack Harkness was surprised to pick up traces of dark matter when scanning an Arcateenian transporter. (TV: Greeks Bearing Gifts)

Behind the scenes
According to REF: Doctor Who: The Visual Dictionary, a non-narrative source, the SS Madame de Pompadour was powered by dark matter.