What would be the opposite of 'luminous'?

The term 'luminous' refers to something that emits or reflects light, implying brightness and radiance. Therefore, the opposite of 'luminous' would be a word that signifies a lack of light or brightness.

'Dull' effectively captures this meaning, as it describes something that is not bright or shining; it suggests a lack of vividness or luster. The other options like 'dazzling,' 'bright,' and 'glittering' all convey brightness or a shining quality, making them the opposite of what 'dull' represents. In contrast to these terms, 'dull' clearly embodies the absence of light or brilliance, establishing it as the correct answer.