A dark data center is a facility that is almost exclusively administered remotely through lights-out management (LOM).
Common tasks that would ordinarily require physical access are automated. Humans have to enter the data center only in extremely rare situations, so there is no need to keep the facility lit.
Benefits of dark data centers include energy and staffing savings. The facility can also be located in more remote areas, which may further lower costs and improve physical security.
A dark data center is also known as a lights-out data center. Data centers that are mostly automated and remotely administered -- but short of qualifying for "lights-out" status -- are sometimes called "dim data centers."