Outer darkness

Jesus uses "outer darkness" (Mt 8:12; 22:13; 25:30) as one of the images for hell. What is that image supposed to conjure in the minds of readers?

The light/darkness, inside/outside contrast may trade on the metaphor of a fortified city. If you arrive after the city gates close at night, you will be stuck outside. You will be exposed to the elements as well as the dangers associated with the night (e.g. nocturnal predators).