Darkness" is a state without light. "Light" is truth from the Lord, so "darkness" represents a state where truth is lacking. Here's a cogent passage from Arcana Coelestia: "The person who has yet to be regenerated is called 'a void and an empty earth', and also 'ground', in which no good or truth at all has been sown - 'void' where there is no good, and 'empty' where there is no truth. Consequently there is 'thick darkness', or stupidity and lack of knowledge about anything that has to do with faith in the Lord and so anything that has to do with spiritual and celestial life. (Arcana Coelestia 17)

257. Since in this prophetical book numbers are often mentioned, and no one can know the spiritual sense of the things contained therein unless it is known what the particular numbers signify (for all numbers in the Word, like all names, signify spiritual things), and since the number "seven" is often mentioned among others, I will here show that "seven" signifies all persons and all things, likewise fullness and totality; for that which signifies all persons and all things signifies also fullness and totality, for fullness and totality are predicated of the magnitude of a thing, and all persons and all things are predicated of multitude. That "seven" has such a signification can be seen from the following passages. In Ezekiel:

They that dwell in the cities of Israel shall set fire to and burn the arms, and the shield, and the buckler, with the bow and with the arrows, and the hand-staff, and the spear; and they shall make a fire with them seven years. And they shall bury Gog and all his multitude, and they shall cleanse the earth seven months (Ezekiel 39:9, 11-12).

Here the desolation of all things in the church is treated of: "those that dwell in the cities of Israel" signify all goods of truth; "to set fire" signifies to consume by evils. "The arms, the shield, the buckler, the bow, the arrows, the hand-staff, the spear," are all things pertaining to doctrine; "to make a fire with them seven years" means to consume them all and fully by evils. "Gog" signifies those who are in external worship and in no internal worship; "to bury them and cleanse the earth" means to destroy all such, and completely purge the church of them.

[2] In Jeremiah:

The widows shall be multiplied more than the sand of the seas, and I will bring to them upon the mother of the youths the waster at noonday. She that hath borne seven shall languish, she shall breathe out her soul (Jeremiah 15:8-9).

"The widows," that shall be multiplied, signify those who are in good and who long for truths, and in a contrary sense, as here, those who are in evil and desire falsities; "the mother of the youths" signifies the church; "the waster at noonday" signifies the vastation of that church, however much it may be in truths from the Word; "she that hath borne seven shall languish, she shall breathe out her soul," signifies that the church, to which all truths were given because the Word was given to it, is to perish; for "she that hath borne seven" signifies to whom all truths were given. This was particularly said of the Jews.

[3] Likewise in the first book of Samuel:

They that were hungry have ceased; the barren hath borne seven, and she that hath many children hath failed (1 Samuel 2:5).

"They that were hungry," who have ceased, are those who long for the truths and goods of the church; "the barren bearing seven" signifies those who are outside of the church, and are ignorant of truths, because they have not the Word, thus the Gentiles, to whom all things will be given; "she that hath many children failing" signifies those who have, from whom will be taken away. In David:

Render unto our neighbors sevenfold into their bosom (Psalms 79:12).

And in Moses:

That the Jews should be punished seven times for their sins (Leviticus 26:18, 21, 24, 28);

"seven times" here signifying fully.

[4] In Luke:

If thy brother sin against thee seven times in the day, and seven times in the day turn again to thee, saying, I repent, thou shalt forgive him (Luke 17:4).

"To forgive seven times, if he should turn again seven times," means to forgive as often as he turns, thus every time. But lest it should be understood to mean seven times, the Lord explained his meaning to Peter, who supposed seven times to be meant, in Matthew:

Peter said, Lord, how often shall my brother sin against me and I forgive him? Until seven times? Jesus saith unto him, I say not unto thee, until seven times, but until seventy times seven (Matthew 18:21-22).

"Seventy times seven" means always, without counting.

In David:

Seven times a day do I praise thee for the judgments of righteousness (Psalms 119:164).

"Seven times a day" means always, or at all times.

[5] In the same:

The sayings of Jehovah are pure sayings, as silver refined in a crucible purified seven times (Psalms 12:6).

"Silver" signifies truth from the Divine; "purified seven times" means wholly and fully pure.

[6] in Isaiah:

The light of the moon shall be as the light of the sun, and the light of the sun shall be sevenfold, as the light of seven days (Isaiah 30:26).

"The light of the sun" signifies Divine truth from Divine good; that "this light shall be sevenfold, as the light of seven days," signifies that Divine truth in heaven shall be without any falsity, thus altogether and fully pure.

[7] In Matthew:

The unclean spirit shall take seven other spirits more evil than himself, and shall dwell there (Matthew 12:45; Luke 11:26).

Here profanation is treated of, and "the seven unclean spirits" with which the unclean spirit would return, signify all the falsities of evil, thus a complete destruction of good and truth.

[8] The "seven times" that were to pass over the king of Babylon have a like meaning, in Daniel:

His heart shall be changed from man, and a beast's heart shall be given unto him, while seven times shall pass over him (Daniel 4:16, 25, 32).

"The king of Babylon" signifies those who profane the goods and truths of the Word; that "his heart should be changed from man, and a beast's heart be given him," means that nothing spiritual, which is the truly human, should remain, but instead there should be the diabolical; "the seven times which were to pass over him" signify profanation, which is the complete destruction of truth and good.

[9] Because "seven" and "seven times" signified all things and fullness, the following commands were given:

Seven days the hands [of Aaron and his sons] should be filled (Exodus 29:35).

Seven days [the altar] should be sanctified (Exodus 29:37).

Seven days Aaron should be clothed with the garments when he was to be initiated (Exodus 29:30).

For seven days Aaron and his sons were not to go out of the tabernacle when they were to initiated into the priesthood (Leviticus 8:33, 34).

Seven times was the altar to be sprinkled for expiation upon its horns (Leviticus 16:18, 19).

Seven times was the altar to be sanctified with oil (Leviticus 8:11).

Seven times was the blood to be sprinkled towards the veil (Leviticus 4:16, 17).

Seven times was the blood to be sprinkled with the fingers eastward, when Aaron went towards the mercy-seat (Leviticus 16:12-15).

Seven times was the water of separation to be sprinkled towards the tent (Numbers 19:4).

Seven times the blood was to be sprinkled in the cleansing of leprosy (Leviticus 14:7, 8, 27, 38, 51).

The lampstand was to have seven lamps (Exodus 25:32, 37; 37:18-25).

For seven days were the feasts to be kept (Exodus 34:18, Leviticus 23:4-9, 39-44; Deuteronomy 16:3, 4, 8).

For the seven days of the feast there was to be a burnt-offering of seven bullocks, and seven rams daily (Ezekiel 45:23).

Balaam built seven altars, and sacrificed seven oxen and seven rams (Numbers 23:1-7, 15-18, 29, 30).

They numbered seven weeks of years, seven times seven years, and then they were to cause the trumpet of the jubilee to be sounded in the seventh month (Leviticus 25:8, 9).

From the signification of the number "seven" it can be seen what is signified:

By the seven days of creation (Genesis 1);

Also by the fact that four thousand men were satisfied by seven loaves and that seven basketful remained (Matthew 15:34-38; Mark 8:5-9).

From this then it is evident what is signified in Revelation:

By the seven churches (Revelation 1:4, 11);

By the seven golden lampstands, in the midst of which was the Son of man (Revelation 1:13);

By the seven stars in His right hand (Revelation 1:16, 20);

By the seven spirits of God (Revelation 3:1);

By the seven lamps of fire before the throne (Revelation 4:5);

By the book sealed with seven seals (Revelation 5:1);

By the seven angels to whom were given seven trumpets (Revelation 8:2);

By the seven thunders which uttered their voices (Revelation 10:3, 4);

By the seven angels having the seven last plagues (Revelation 16:1, 6);

And by the seven vials full of the seven last plagues (Revelation 16:1; 21:9);

and elsewhere in the Word, where "seven" is mentioned.