'Darkness' in the Bible

For God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ.

Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?

and what fellowship to light with darkness? and what concord to Christ with Belial? or what part to a believer with an unbeliever?