Junk was a lifeless rock of a planet that the Trade Federation used as a scrapyard.
Originally they intended to begin a recycling program, but those deals fell through.
Junk's atmosphere has very little oxygen and moisture, which kept the scrap in reasonably good condition.

During the Revolt it became the crash site of a Quintession ship.
The ship was a small high speed transwarp vessel carrying several scientists and the Creation Matrix.

The Creation Matrix was lightly damaged and began randomly manufacturing Junkions, the Cybertronian-like mechanoids that now inhabit Junk. The clones created by the Junkions may have their Facsimile Sparks converted into Sparks by the Creation Matrix.