The ninth plague on Egypt was the plague of darkness. It was not normal darkness like that of an eclipse. It was a supernatural darkness, "even a darkness which may be felt" (Exodus 10:21). The darkness persisted for three days. Not even artificial lights such as lamps and torches could pierce the blackness. The Egyptians stayed indoors for the three-day duration. But the Israelites had light.
The plague of darkness symbolized a defeat of the Egyptian sun god, chief over the pantheon. It also symbolized the spiritual darkness of Egypt. Though the Israelites were the slaves and the Egyptians the masters, the plague of darkness illustrated that it was the Egyptians who were in servitude. They were enslaved to the adversary and their false gods. They were under the dominion of the kingdom of darkness. Though the Hebrews were slaves, they were spiritually free. As servants of the truth, they were part of the kingdom of light.
A person in spiritual bondage might not know (or will not admit) that he is in bondage. He feels as if he is in control of his life, calling the shots and making decisions, but ultimately, he serves a great emptiness.
For example, a man with an alcohol addiction tells himself that he could stop at any time, and that he only needs a drink to "take the edge off." It is obvious to everyone around him that he has a compulsive disorder and his alcohol consumption is out of control, but the man continues to deny the problem.
In the same way, human beings without faith and without God live in a state of denial. They refuse to admit that a great vacuum exists within them. Their souls are starving for light, but they don't know it, nor do they know how to feed it.
The Apostle Paul tells us to give "thanks to the Father, who has qualified us to share in the inheritance of the saints in Light. For He rescued us from the domain of darkness, and transferred us to the kingdom of His beloved Son, in whom we have redemption, the forgiveness of sins" (Colossians 1:12-14).