A planet that is made up of dark matter it would be extremely dense and that has good mining little artifact and lots of research also sinse it is the one of the most abundant types of matter in the universe it would be only found large or bigger due to the density of the planet and the fact that dark matter is very very hard to detect would make it even more difficult to find than a terra and just below a dyson.

This one of the more not odd scifi type of planets suggested.

I was thinking of a mission chain that would have a shadow based race and shadow planets sounds like something with Exotic matter.

I thought Dark matter was completely different from matter that actually makes up planets and all the physical universe we interact with.

It is. There are no planets made of it. It is probably not matter at all, but gravity bled through close lying branes.

Ehem, in Futerama There was a planet like a Jungle type kinda like exotic, but it had dark matter as the mantle and outercore with a normal inner core, they mined it because its used as rocket ship fuel.
So A planet made out of darkmatter might not be a good idea but on the right track

Dark matter is the theoretical "matter" that makes up space. Space being the...er...space between celestial bodies. I'm not an astronomer or a physicist, so correct me if I'm wrong.