What is Dust?

Dust is a by-product of the pure Unobtanium used by the ships engines which ends up circulating in the atmosphere as rematerialised tiny particles and is recovered by the Arch. Dust is described as a nearly inert isotope of Unobtanium and is produced in an identical form by the engines of all species. So the dust produced by one species can be converted by another species into fuel for their own needs.

Also a Currency

Dust is one of the main in-game currencies, in fact its flexibility and ability to be converted back into Unobtanium has made it the natural currency of the galaxy.

We know that Ship of Heroes is going to have an Auction House in Apotheosis City; the building already existed when the first SoH Youtube video was posted by Heroic Games though there was nothing actually in the building at the time. So dust will presumably be used there as a local currency between players as well. Whether or not there will be NPC's in game selling items and services for dust remains to be seen.