Unedions are unreasonably tiny particles that somehow manage to exist, as they're FAR smaller than anything that came before them. Even a Milion's size compared to Metium is nothing compared to an Unedion compared to a Milion size wise.
The Glow in the image is a (Tiny), not the Unedion itself. Their barrier is usually invisible, which is why you can't see it in the image. Unedions are usually quite full, as they're mostly filled with much smaller things. They also have several Mini-Unedions.
How they remain stable[]
They remain stable because of their barrier, an without them they'd turn into a mess mainly because of the (Tiny) that exists within them, that thing being way too unstable.
Mini-Unedions[]
Mini-Unedion's are smaller variants that aren't that different from the regular ones, except that they dont have a (Tiny) within them because they wouldnt fit.
Overall[]
Unedions are very small particles that are quite full because of the (Tiny) in them and they're stable because of their invisible barrier.
The Nonon Chain (Class -2)
Nonon->Metium->Milion->Unedion->(Tiny)->-
(You are at Unedion)
The Chain is Now Complete.