I've long been thinking of many uses for dark matter:
1. I've been wanting to do a campaign set on a Big Dumb Object, probably some sort of ringworld. But I have an idea to use dark matter to reinforce the structure by essentially providing the equivlant of pilings to support the super long arch.
2. I've been thinking of a world where there is a dark matter power reactor that operates inside the planet, generating large amounts of power that is practically undetectable, but able to effect things on the planet.