The answers to the following clues will have something in common:

1. Low point
2. Africa, once
3. Show up, unwanted
4. Feign confidence
5. It's never been seen from earth
6. 500-year period of cultural decline
7. Uninformed
8. Unexpected political candidate
9. Astrophysics postulation
10. Photographer's need, once
11. Evil - informally
12. Wild guess
13. History of misdeeds

Answers:

1. Darkest hour
2. Dark continent
3. Darken the door
4. Whistle in the dark
5. Dark side of the moon
6. Dark Ages
7. In the dark
8. Dark horse
9. Dark matter
10. Darkroom
11. Dark side
12. Shot in the dark
13. Dark past

Credit: LA Times Crossword
Sylvia Burstzyn and Barry Tunick (Idea and #1-7)
LIGHTS OUT