When the Optical Fiber transmits it's pulses of light the Dark Fiber becomes Lit Fiber.
The fiber optics network was over built. This has lead to many fibers in the network that are currently not in use. In fact there are many fibers that have never been used. A fiber that is in use transmits light pulses and is called lit fiber. A fiber that is not in use is called dark fiber.
Dark fiber exists because it was found that it cost almost the same price to install large fiber cables as it did to install smaller cables. Due to the potential for future savings, telecommunications companies installed many dark fibers in each cable that was used. Many of these fibers are not being used even now.
As the fiber optics networks were being laid, it was discovered that the cost of the fiber cable was very small as compared to the cost of building the conduit and installation. As an example, the cost of laying a cable with 100 fibers was the same as a cable of 10, 000 fibers. As a result, telecommunications companies, would lay larger cables whenever building the network.