"What is the opposite?"
Translation:Hvad er det modsatte?
No. Here, modsatte is an adjectival noun. This means its an adjective being used as a noun. So the front article form of the definite must be used (den/det/de), rather than the end article form (en/et). Modsat/modsatte are the common/neuter genders, as can be found in a dictionary (e.g. ordbog.dk). You'll note there is no modsatten/modsattet that might correspond to an end article. Nouns formed from other word classes (e.g. adjectives, pronouns, interjections) are almost always neuter, So the choice of den/det for common/neuter should match. Thus: det modsatte.
Here, it is an adjective acting as a noun. Therefore, it must still be inflected like an adjective.
It's the same as:
A tall man = En høj mand
The tall man = Den høje mand
The tall (one) = Den høje