My favorite smart ass response when someone asks me what dark fiber is:
Evil Fiber
But seriously, it is just a term of art, when one carrier sub-leases fiber without any terminating equipment and they provide their own equipment to light the fiber. It is "dark" from the standpoint of the provider, they are not "lighting" it. The customer is. Typically the fiber is leased using something called an IRU indefeasible right to use. Basically "condo-ised" fiber.
Don't forget the "dark" web, wharever that means anymore. These terms are great for marketing groups to paint an image but often very quickly lose any technical relationships they once had.
Ooooooh. I thought "dark" was some weird domain-specific nomenclature to describe "not available on the open market", with the open market being like a transport network of (lit up) lines, and dark = not lit up = not on the open market.
Now I actually understand the definition I'd say "unlit fiber" feels like a better-resolved way of putting it, although I could see the uninitiated imagining that describing a service that needed an additional subscription on top or something, which isn't quite the right nuance.
More or less, someone with a cable leases you dark fiber, and then you light it. What are you using? lit fiber (obviously), what did you lease? dark fiber.
When it's severed, it's not exactly lit anymore either ;)
As I recall, "dark fiber" came into use after the dotcom bust left lots of overbuilt and unused network infrastructure around that companies could buy up for years after for pennies on the dollar. Buying "dark fiber" in that context had meaning - it meant you were buying already built-out and unused fiber, compared to running your own fiber lines at full cost as had previously been more common.
Well, I was joking a bit, but I would consider it dark fiber when you lease the fiber and run the equipment on both ends, and lit fiber when the owner of the fiber is running the equipment on both ends and selling you IP transit.
But seriously, it is just a term of art, when one carrier sub-leases fiber without any terminating equipment and they provide their own equipment to light the fiber. It is "dark" from the standpoint of the provider, they are not "lighting" it. The customer is. Typically the fiber is leased using something called an IRU indefeasible right to use. Basically "condo-ised" fiber.