Dark fiber - which is also known as unlit fiber or black fiber - is an unused optical fiber that has been laid. It is usually used in Telecom and Network Communications, and there are thousands of miles of unused dark fiber cables across the US. While it is currently unused, it's known to be "dark" as no light pulses are being transmitted through it. In normal fiber cables, light pulses send information, but it's essential that these cables are instead taken advantage of. With FieldEngineer.com, you can utilize our dark fiber map solutions which are customized to help 
Today the term dark fiber is used to discuss the ever-growing, popular procedure of leasing out fiber optic cables from a network provider/service provider, or, out to the Fiber installation/fiber infrastructure that isn’t owned by regular carriers. Dark Fiber can still be called dark, even if it has been utilized by a fiber lessee and not by the owner of the cable.

More info: What is Dark Fiber