Huon

Huon particles were a type of particle, as well as energy, that existed during the Dark Times of the universe. Huon particles attracted each other, even over long distances.

The Dalek Time Squad once encountered the abandoned ark of a species wiped out by the Kotturuh's judgement. The ship was packed with Huon particles that the Daleks tried to harvest. The Eighth Doctor revealed that this was a trap that would ensure the right people would find and harness their knowledge whilst those who boarded with greedy intentions would be met with greed. (COMIC: Tales of the Dark Times)

On Gallifrey, Huon particles were used in experiments conducted into time travels, but they were primarily utilised by the Racnoss in reviving their new-born, giving them a dose with their first meals, especially when they had been in hypersleep, (PROSE: The Labyrinthine Web) in huon pulses, (AUDIO: Empire of the Racnoss) and to power their Webstars and other technology. The Time Lords destroyed the particles near the end of their war with the Racnoss due to their deadly nature, the only known remaining concentrations of Huon particles existing in small amounts within the heart of the Doctor's TARDIS. (TV: The Runaway Bride)

Torchwood One recreated the particles in liquid form using equipment at the Torchwood Institute, (AUDIO: In the Shadows) using a hydrogen base. These inert liquid particles needed to be activated in a living being, such as a human. They could be mixed with other liquids and imbibed, slowly poisoning the victim. This process could take as long as six months or as little as a few minutes, depending on the amount consumed. Once activated, the Huon particles would start to glow and become magnetised, attracting and being attracted to other Huon particles. If enough were absorbed, it could even pull other Huon particles from different time zones (such as the TARDIS). As such, bio-dampers were ineffective against hiding Huon particles. (TV: The Runaway Bride)

In 2007, the Empress of the Racnoss attempted to use the Huon particles in Donna Noble to wake the hibernating young Racnoss at the core of the Earth, but they were drowned by the Tenth Doctor. (TV: The Runaway Bride)

When the Doctor's TARDIS was destroyed in an event level chronal distortion, the Tef'Aree could not transport Emily Winter out of the fifth dimension to any planet as the Huon radiation was too strong. They instead returned her to the TARDIS a few seconds before it was destroyed, enabling her to prevent it. (COMIC: Tesseract)

Jack Harkness, recalling something that the Doctor told him, speculated that Racnoss Huon particles may have resulted in the Blessing, but was forced to admit he had no idea what caused it. (TV: The Blood Line)

Behind the scenes
Since Huon is described as both an electromagnetic wave and a particle in The Runaway Bride, this makes it an example of, acting as a photon does in the real world.