Dust was an elementary particle. It had different names in the different worlds of the multiverse: Dust or Rusakov Particles in Lyra's world, Shadows or dark matter in Will's world, and sraf in the mulefa world.
Dust was an elementary particle which was conscious, and which conferred consciousness upon certain species, such as humans and mulefa. It was attracted especially to adults and objects created by sentient species.
Mulefa were able to see Dust with the naked eye, and as such they were able to identify other sentient and intelligent creatures. Humans were unable to see Dust with their own eyes, but could use technology such as the amber spyglass or a special emulsion used to develop photograms to view it. Humans developed ways of communicating with Dust, such as the alethiometer, the Cave and the I Ching.
The witch, Serafina Pekkala, would float on her cloud-pine at night and listen to the whispering that was made up of silence. It was a different kind of silence however and she later realised it was that of Dust.
Research by the Dark Matter Research Unit at the Oxford University in Will's world estimated that Dust first came into contact with humans around 33,000 years ago. In Lyra's world, the particles were first discovered by the Muscovite experimental theologian Boris Rusakov and they were named after him.
With the invention of Æsahættr in the world of Cittàgazze in the 17th century, Dust began to leak out of the windows which were cut. It was also around this time that the alethiometer was invented in Lyra's world. The Church began to study Dust, believing it to be the manifestation of Original sin.
The flow of Dust was worsened when the Magisterium's bomb created a massive window into the Abyss, and was slowed when Lyra Silvertongue and Will Parry kissed. After the teenagers returned to their own worlds, angels closed the windows between worlds to stem the flow of Dust.