'Darkness' in the Bible

Then I saw that wisdom excels folly, as far as light excels darkness.

The wise man's eyes are in his head, and the fool walks in darkness -- and yet I perceived that one event happens to them all.

All his days he also eats in darkness, he is frustrated, and has sickness and wrath.

for it comes in vanity, and departs in darkness, and its name is covered with darkness.

Yes, if a man lives many years, let him rejoice in them all; but let him remember the days of darkness, for they shall be many. All that comes is vanity.