I have often been asked by people what Dark Fiber is, my answer is usually that it is evil fiber. Just kidding. Seriously, dark fiber is quite simply fiber that does not have the usual network equipment attached to it and hence, no lasers are shooting photons through it, thus the term "dark" meaning not being lit by lasers.<p>The largest cost of laying fiber is not in the fiber itself, the glass itself is relatively cheap in comparison to the cost of obtaining the right of way, the labor and so on. Often municipalities, counties, states or whatever charge a fee for these rights of way if the fiber is underground (the most expensive). If they are attaching aerial fiber to existing poles, the pole owners, often power companies, have also paid for the rights of way and further charge a poll attachment fee. Regardless, all the other cost typically far over shadow the cost of the individual strands.<p>Typically a carrier will install a larger fiber count cable since the incremental cost of installing a 144 count fiber rather than a 72 count for example, would probably be relatively minor so why not have a few extras. Most carriers have the choice as to whether or not to sell off (typically using an instrument called an Indefeasible Right to Use, which is sort of like a condo but for fiber, usually long term, 20years). Usually they will sell or trade with other carriers to help offset costs, for example carrier A might trade 4 fibers in San Francisco for 4 fibers in LA for particular routes. In that way they are not so directly aiding a competitor but also gaining routes for themselves.<p>Usually, carriers do not like to sell off DF to customers such as large enterprises because quite simply, those customers would never have to buy anything from them again because they could continuously gain more capacity by placing DWDM equipment on each end (not endlessly of course but for all practical purposes for most customers, endless).<p>That is why most carriers like Century Link or others (especially ILEC's, Incumbent Local Exchange Carriers, i.e. the phone company) do not like selling DF. Typically these carriers have purchased a Franchise agreement with cities or whichever authority. These franchise agreements are not cheap (usually 5% of gross or something like that, basically it is like a tax) that gives them in theory access to all rights of way from the controlling body (cable co's do the same thing).<p>Because of that they maintain often a de-facto monopoly or a powerful market position that they do not want jeopardized by the availability of DF which would provide extremely cheap bandwidth.<p>I have often thought that the smart thing for cities to do, if perhaps one were to be able to start from a green field, would be for the city to place telecom conduit up and down every street from the get go and then rent out inner-duct space to carriers. That would make it easier for competitors to access the rights of way without all of the disruptions.<p>But perhaps new mmwave 5G technology will eliminate such concerns eventually (but I am skeptical).