Although the title sounds a little bit on the ominous side, a dark fiber network simply refers to an internet connection which is established using previously idle fiber-optic cables.
Oftentimes, when an internet service provider is planning to introduce fiber internet services to an area, they lay more cable than is generally necessary in order to save on the cost of adding extra infrastructure later. If these cables do indeed remain unused, they are referred to as dark fiber, which simply means that they are fiber-optic cables with no activity.
If these strands remain unused for long enough, internet service providers will sometimes lease these dark fiber cables to individuals, or to other companies who wish to use them to establish a network. Once they’ve been leased out, the fiber optic cables are no longer controlled by the company which originally owned them, and the lessee must provide the necessary infrastructure to enable an internet connection.
Since fiber-optic cables are also used by phone companies, it is also possible to lease excess connections which were not originally intended to be used for the internet. These phone company connections work just as well as the cables laid down specifically for internet purposes.