DWDM (Dense Wavelength Division Multiplexing) carries dozens of “colors” of light (wavelengths) simultaneously on a single fiber pair — typically 40 to 96 channels, each bearing 10, 100 or even 400 Gbps. One dark fiber pair becomes the equivalent of dozens of circuits.
Three ways to benefit: lease a lambda (one wavelength) from a carrier — the middle ground between an Ethernet circuit and dark fiber; equip your own dark fiber with DWDM (affordable passive muxes, or active chassis); or ignore it and buy managed service.
The king use case: inter-data-center links of a BCP/DRP pair — DWDM carries storage replication, WAN and management on separate channels, with layer-1 encryption possible, without re-leasing fiber at every bandwidth upgrade.