The Latin word for darkness is tenebrae. That's actually plural. Why don't we use the singular form in this case?
the Latin noun “tenebrae” is a “plurale tantum nomen”(literally, “plural only noun”), i.e. a noun that appears only in the plural form and does not have a singular variant for referring to a single object, such as e.g. “tenebrae” (darkness), “deliciae “(delight), “fauces” (throat), whose singular form is obsolete.
So, the reason why we cannot use the singular form “tenebra” (nominative singular) instead of “tenebrae” (nominative plural) is that this noun is plural by signification in Latin as it refers to a whole of dark colours.
In short, in Latin some nouns are commonly or exclusively found in the Plural (pluralia tantum ), just like some nouns in English (see e.g. “politics” and “statistics”).