How come dark-ghost types have no weakness?

Question

I'm wondering because I don't get it. I know with dark, normal type is covered for ghost, but what about dark?

P.S. I know this is a dumb question, but I'm not sure on what it means.

Comments

As of the time Ninja's answer was posted, it was correct, but I would like to say, for the purpose informing people that click on this question, curious of what the answer is:

Ghost/Dark-type Pokemon no longer have no weaknesses, due to the introduction of the Fairy-type in generation 6. Fairy-type moves are super effective against Dark-type Pokemon, but neutral against Ghost-type Pokemon, making Dark/Ghost-types weak to Fairy.

Answer 1

Think.
http://pokemondb.net/type/dual
These types have no effect on Dark pokemon:
PSYCHIC
These types are not very effective against Dark pokemon:
GHOST DARK
These types are super-effective against Dark pokemon:
FIGHTING BUG

These types have no effect on Ghost pokemon:
NORMAL FIGHTING
These types are not very effective against Ghost pokemon:
POISON BUG
These types are super-effective against Ghost pokemon:
GHOST DARK

Ghost cancel's dark's weakness. Ghost and dark aren't very effective on dark, but super effective on ghost, thus making it even. Bug isn't very effective on ghost, but super effective on dark, thus making it even. Fighting is super effective on dark but has no effect on ghost, thus canceling it out.

Comments

3 minutes ahead, but you put more info. Good answer, bud.

---

Was finished typing and your answer was there.

---

It sucks how Spiritomb has a horrible movepool :(

Answer 2

Ghost is vulnerable to: Dark, Ghost
Dark is vulnerable to: Fighting, Bug

Ghost resists/is immune to Durk's weaknesses.
Dark does vice versa.