Dark fiber is one without internet traffic or that’s currently not “lit”. This is similar to an electrical wire that doesn’t contain electricity. Network equipment must be specially designed to be connected to both ends the fiber for an efficient distribution of bandwidth throughout the building.
Like a faucet for water the service providers who are the owners of the fiber will “throttle down” the speed of the internet in your premises, much like the flow of water from the tap. If you have or lease the specialized network equipment, the Internet tap could be opened up to full. The amount of data is decreased by the service providers in order to meet the amount agreed upon.
Dark fiber is the property of anyone other than the lit service provider. But, it is important to also consider all costs of the installation, maintenance and setup. Otherwise, the service provider will take care of the other aspects.