Synonyms of the word

lightless

• adj. Lacking light; unilluminated.

dark

• adj. Having an absolute or (more often) relative lack of light.
• adj. (of colour) Dull or deeper in hue; not bright or light.
• adj. Hidden, secret, obscure.
• adj. Without moral or spiritual light; sinister, malign.
• adj. Conducive to hopelessness; depressing or bleak.
• adj. Lacking progress in science or the arts; said of a time period.
• adj. With emphasis placed on the unpleasant aspects of life; said of a work of fiction, a work of nonfiction…
• n. A complete or (more often) partial absence of light.
• n. (uncountable) Ignorance.
• n. (uncountable) Nightfall.
• n. A dark shade or dark passage in a painting, engraving, etc.

invisible

• adj. Unable to be seen; out of sight; not visible.
• adj. Not appearing on the surface.
• adj. (Internet) Apparently, but not actually, offline.
• adj. (psychology) That is ignored by a person.
• v. To invisiblize.
• n. (obsolete) An invisible person or thing; specifically, God, the Supreme Being.
• n. (obsolete) A Rosicrucian; so called because avoiding declaration of his craft.
• n. (obsolete) One of those (as in the 16th century) who denied the visibility of the church.

unilluminated

• adj. Not illuminated.

unlighted

• adj. Not lit.

unlit

• adj. Not lit.

unseeable

• adj. invisible.
• adj. unimaginable.