Spatial Rift

Tears in space. An analogy often used in Einstein's theory of general relativity is balls on a trampoline. Each ball creates a divot in the trampoline (gravity), causing nearby balls to attract each other. The heavier it is, the deeper it sinks into the trampoline.

A Rift can be modelled as a tear in the trampoline.

As such, anything that makes contact with a rift 'falls out of the world into oblivion'

However, Rift-Capable Warp Drives (RCWDs) can cross rifts safely by repairing space around the drive temporarily.

Manifestation

Rifts would not emit light, nor would they reflect light.

Localization

Rifts can be created by certain types of superweapons or extremely powerful cosmic events. They are extremely difficult to repair, requiring the most cutting edge technology just to fix a small area tempoarily.