Darkness or deep gloom is a place outside of God’s presence, likely some kind of underworld. The angels are being kept in a place until the day of the Lord (Zephaniah 1:14; Malachi 4:5; Acts 2:20; Revelation 16:14) when he will come to finally throw them into the fires of hell (Revelation 20:10).
6 And the angels who did not stay within their own position of authority, but left their proper dwelling, he has kept in eternal chains under gloomy darkness until the judgment of the great day