What is meant by the expression "living in darkness"? At my church (Calvary Chapel) we have guest speakers from time to time. Many of them mention the same phrase "living in darkness", refering to the time before they were saved.

Are there Biblical (or more specifically New Testament) references to this concept and a description of what it entails or is it just a relative and subjective phrase coined by man?
Variants of the phrase appear several times in 1 John

1 John 1:5-6 (ESV)
5   This is the message we have heard from him and proclaim to you, that  God is light, and in him is no darkness at all. 6   If we say we have fellowship with him while we walk in darkness, we lie and  do not practice the truth.
1 John 2:9-11 (ESV)
9  Whoever says he is in the light and  hates his brother is still in darkness. 10  Whoever loves his brother abides in the light, and in him there is no  cause for stumbling. 11  But whoever hates his brother is in the darkness and  walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes.

Darkness is contrasted with the picture of God as light, and describes living in sin without knowing or seeing God.