Dark fibre is fibre optic cable that has been laid for use by telecoms networks, but which is not used. There are a number of reasons why fibre may remain dark:
- ill-judged over-investment in capacity (this was particularly bad during the dotcom boom)
- deliberate planing for future growth.
Deliberate building of excess capacity happens because of the difficulties involved in laying fibre optic cables: digging up the ground (or even worse, running cables through oceans) is very expensive, so there is every reason to lay more than you need immediately, in order not to have to do it again in the near future.
Dark fibre is sometimes sold, the buyers presumably having plans to make it "lit".