Dark fiber is a strand of optical fiber leased with no active equipment: you light it with your own transceivers — 10G, 100G, or dozens of wavelengths in DWDM on the same pair. Bandwidth is limited only by your optics.
Strengths: near-unlimited capacity at fixed cost, minimal latency, full control (layer-1 encryption possible). Constraints: you operate it (monitoring, spare optics), no contractual bandwidth guarantee as with carrier Ethernet, and a price that depends entirely on the route (per km, metro).
Typical use case: linking two data centers in the same metro for synchronous replication — below ~60 km, dark fiber or an inter-site cross-connect often beats an Ethernet circuit; beyond that, the carrier service regains the advantage.