Jude 1:6 from the Revised Version reads, "And the angels which kept not their own principality, but left their proper habitation, he hath kept in everlasting bonds under darkness unto the judgment of the great day." This is referring to the angels in Noah's day, who left heaven (their proper habitation) and married the women to create a hybrid race. In Gen6:7, the LORD says, "I will destroy man… both man and beast…" in the flood. Consequently, the entire hybrid race perished. However, the angels did not drown. Jude informs us God has put them in bonds under darkness until the judgment day.
Was the darkness spiritual or physical? It is probably both. Spiritual truths are given to those who love God. "For the Lord GOD does nothing without revealing his secret to His servants the prophets." (Amos 3:7) "The secret of the Lord is with those in whose hearts is the fear of him; he will make his agreement clear to them." (Psa.25:14) These scriptures clearly state God will reveal His plans and purposes to His servants and to those who fear Him. In fact, I Peter 1:10-12 explains, "Concerning this salvation, the prophets who prophesied…inquired carefully…in the things that have now been announced to you…by the Holy Spirit sent from heaven, things into which angels long to look." God reveals His purposes in His due time and even the angels are looking into these fulfillments! Fallen angels have not been faithful servants nor do they fear God. Consequently, their spiritual understanding is darkened.
What could bonds under (physical) darkness represent? Before the flood, angels openly materialized with fleshly bodies. After the flood, we do not see fallen angels (demons) materializing and marrying people. It may be suggested God will never permit demons to impersonate people anymore. The devils manifest themselves in séances, in black magic, through mediums, through demon-possession, but not openly in daylight with human bodies. God has restricted them to work in darkness.