The Trans-Kuiper belt (also called the Zhang belt) is a large collection of comets and dust orbiting 100 AU away from the sun.
The Trans-Kuiper belt was formed during the formation of the Sol system 4.6 billion years ago. It is comprised entirely of material from the protoplanetary disk and has proven invaluable in the study of the early solar system because of this. It never coalesced into a planet due to the low density of the belt.
The belt was discovered accidentally by a research team in Hong Kong looking for the Ninth Planet. The team discovered hundreds of objects exactly where they were theorized to be according to the Large Disk Theory (a scientific theory describing the Trans-Kuiper belt before its discovery).