The blessed state of the good is contrasted with the awful state of the wicked. The wicked are in “outer darkness.” This phrase has two referents. The first is the light/dark contrast that is frequent in the scriptures. Light is life and good, dark is death and evil. Thus “darkness” is the presence of evil.
The second reference is inner/outer. This is a reference of distance. While the reference of “outer” is not completely certain, we may suppose that it is related to the distance from the source of light. The further one goes from the source of light, the darker it becomes, and “outer” darkness would be a far distance from the source of the light. While the wicked may still be in the realm of God, the are clearly not in his presence, nor close to his glory.