Dust in Philip Pullman's trilogy of novels His Dark Materials is a fictional form of dark matter, an elementary particle that is of fundamental importance to the novels. Dust is invisible to the human eye and cannot be seen without the use of special instruments such as the amber spyglass or a special film. However, while humans cannot see dust without the use of outside devices, creatures such as the mulefa are able to see dust with their own eyes.
Unlike ordinary particles, Dust is conscious. It falls from the sky and is attracted to people (especially adults) and wears of onto objects made by people. This makes it of great interest to the Church, which believes that it may be the physical manifestation of Original Sin. It is later learnt that Dust actually confers consciousness, knowledge and wisdom, and that Dust is formed when matter becomes conscious. This allows creatures who have the ability to see dust to identify other sapient and intelligent creatures. An example of this is when the mulefa are able to distinguish Mary Malone as an intelligent being, because of the dust surrounding her, when compared to the other animals in the mulefa's world.
Dust is also the thing that allows all "magic" to be done in the worlds. Those beings that can understand Dust by looking at it can also see the truth in things and are able to change things by getting into a certain state of consciousness. An example would be the panserbjørne, who are able to see the truth in all things and therefore cannot be tricked, unless they become like humans. There are also the witches who understand Dust on such a level that they can use it to make them fly, cast "spells", make themselves unnoticed, and many other things.
Dust is also the thing that connects humans to their dæmons. This being is the physical manifestation of the soul that can talk and is in the form of an animal. It sends the Dust to the human to allow the human consciousness. Even in worlds that don't have apparent dæmons, they still exist, though they typically do not take the form of animals. In some other worlds one's dæmon is one's silent consciousness in the back of one's head, that other voice that confers intuition. If the bond between a child and their dæmon is severed (as through Intercision), both the child and the dæmon typically die, though not always, and rarely immediately. If the separation occurs after Dust has settled on the person (that is, after he or she has reached adolescence), the person simply becomes a lifeless shell.
It is Dust that provides the answers given by the alethiometer, the I Ching system of divination and also the computer that Dr. Mary Malone creates in order to communicate directly with these particles by using one's consciousness.
Dust has various names among the various different worlds within the trilogy. Dust was previously known (in Lyra Belacqua's universe) as Rusakov particles after their discoverer, Boris Mikhailovitch Rusakov. It is known also as Shadows in our world (Pullman relates Dust to dark matter), and the mulefa's word sraf accompanied by a leftward flick of the trunk (or arm for humans).
Angels are formed when Dust condenses. Even The Authority came into being this way. However, Angels are not in reality the human-like figures they appear to be. They are the physical manifestation of spirit making something 'be'. Dust is discovered, by Dr. Malone, to actually be Angels and consciousness. Because consciousness is the thing that makes us "sin", it can (in theory) be seen as original sin. This is the point of view seen by the Magisterium, and therefore they seek to destroy it. However, they fail to see what the full repercussions of this would be, as they are ignorant to the true nature of Dust. Eliminating it would mark the end of consciousness, and would most certainly lead to the quick destruction of all worlds with conscious life in the multiverse.
There is a story in Genesis in which God creates Adam and Eve from dust, and in the first book of this series, Northern Lights, Lord Asriel shows the Bible to Lyra: "In the sweat of thy face shalt thou eat bread, till thou return unto the ground; for out of it wast thou taken: for dust thou art, and unto dust shalt thou return". Asriel also says that some Church societies argue that it should not be "'and unto dust shalt thou return,'" but, "'and thou shalt be subject to dust.'" In chapter 12 verse 1, the letter to the Hebrews says, "“let us throw off everything that hinders us, and the sin that clings (entangles us) so closely”. It may also be interesting to note that in the Paradiso by Dante Alighieri, pp.11-12 'Dust' is used in reference to the Human Body: "And even as the soul within your dust..." Beatrice tells Dante that just as the soul distributes its power throughout the physical body, where it is differentiated according to the part that receives it, so too is the power of God distributed throughout the Heavenly Spheres and the undifferentiated power of God becomes differentiated when it combines with the qualities associated to the Spheres,(Theologians for the Sun, Contemplatives for Saturn, etc.) creating a "mingled virtue".