Lightness/Brightness:
* Light
* Bright
* Luminous
* Radiant
* Illuminated
* Gleaming
* Shiny
Color:
* White
* Pale
* Bright
* Colorful
Mood:
* Cheerful
* Joyful
* Happy
* Optimistic
* Hopeful
Clarity:
* Clear
* Transparent
* Visible
* Understandable
Knowledge:
* Enlightened
* Informed
* Aware
* Knowledgeable
Evil/Darkness:
* Good
* Pure
* Holy
* Virtuous
Other:
* Day (as opposed to night)
* Open (as opposed to closed)
* Shallow (as opposed to deep)
It's important to consider the specific context when choosing an antonym for "dark". For example, if you're talking about the darkness of a room, "bright" would be a good antonym. But if you're talking about the darkness of a person's soul, "good" or "virtuous" might be more appropriate.