[quote=@ARedneckWizard]
Dark matter is a good analogy but I was thinking more along the line of a primordial plasma or Ylem I think it's called. Or we could reflavor it as entropy/decay magic. 

I've got [i]some[/i] ideas for the cave elves.
[/quote]

Actually, I'm going to have to rule that dark matter and primordial plasma ([i]i.e. ylem[/i]) are the same thing. The humans and basically everyone on the surface sees it as "dark matter". While the Cave Elves, being the ones who actually know how to harvest and contain it, they refer to it as "ylem". They understand the true workings of the substance, hence why they have been able to name it. Others don't understand its true nature, so they have a more vague way to describe it.