Dark fibre is normal installed optical fibre that is not lit. It is not a new and different type of fibre!
It is just that the responsibility for the equipment at each end that lights up the fibre is different. So with dark fibre, you get access to a continuous optical circuit (usually a pair of fibres), that provides a direct connection between the two end points. You are then responsible for lighting up that fibre with your own equipment. This gives you full control of the link and of the traffic that you put over it.
For example, it is relatively straightforward (if you know what you're doing) and cost-effective, to use this dark fibre to carry multiple 10Gb/s channels using different wavelengths of light over the same pair of fibres. For modest distances (up to 40km) you could probably send up to 18 inexpensive CWDM channels, for longer distances and more capacity you can use DWDM technology and fibre amplifiers to send 80 or more channels over multiple 80km spans. If you need even more capacity and have plenty of money to spend then each channel could operate at 100Gb/s.
This compares with a traditional telecoms service, say a 1Gb/s Ethernet point-to-point 'leased line', where the telecoms service provider runs fibre into your building and connects that fibre back to their central office or telephone exchange. They then install their equipment in your premises that takes the 1Gb/s Ethernet signal from your network and squirts it down the fibre to their network equipment where it is aggregated with the rest of the traffic on their network, directed towards your remote location and handed back to you as a 1Gb/s Ethernet signal. Each time you want more capacity you have to pay for another leased line.
Become a Certified Optical Network Associate and learn to light up dark fibre. Available from OTT partners in UK, Ireland, USA and Africa