This outer darkness probably refers to the temporary holding place for the wicked and those who don't know Christ prior to the judgement, also known as prison or hell (not to be confused with the final resting place for the sons of perdition, those not redeemed by Christ's Atonement).
The following scriptures shed more light on the nature and purpose of this holding place:
In the parable in Luke 16:22-23, a rich man ended up in this place of torment after his selfishness allowed a beggar die:
22 And it came to pass, that the beggar died, and was carried by the angels into Abraham’s bosom: the rich man also died, and was buried;
23 And in hell he lift up his eyes, being in torments, and seeth Abraham afar off, and Lazarus in his bosom.
In 1 Peter 3:19 we read that Christ preached to these spirits in prison after his resurrection:
19 By which also he went and preached unto the spirits in prison;
In the next chapter, 1 Peter 4:16, we read that the gospel was preached to these dead in prison so that they will have the chance to accept it and be saved.
6 For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
Zechariah 9:11 teaches that (at least some of) those in prison will be saved by Christ's Atonement.
11 As for thee also, by the blood of thy covenant I have sent forth thy prisoners out of the pit wherein is no water.
At some point all those in this hell will be brought out and judged before being consigned to their final destination. Revelation 20:13-14:
13 And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works.
14 And death and hell were cast into the lake of fire. This is the second death.
Notice that it was only after judgement that death and hell were cast into the lake of fire. This "second death" is permanent separation from Christ.
