Some adjectives can also be used as nouns :
a) to denote an abstract quality - the theatre of the absurd ; the good of the environment; the realm of the unknown.
Adjectival nouns describing abstract qualities are grammatically singular : The unknown is scary!
b) to denote a class of people - the rich, the poor, the unemployed, the overweight, the Left. Nationality adjectives ending in /s/ /z/ /ʧ/ or /ʃ/ may often be used in this way : the British, the Chinese, the Danish, the Dutch, the English, the French, the Irish, the Japanese, the Swiss, the Welsh, etc.
Adjectival nouns describing a class of people are grammatically plural : Information on who the unemployed are—and who they are not—can provide valuable information about...