主流 works the same way as 'mainstream' in English: 主流文化 (mainstream culture), 主流社会 (mainstream society), 非主流 (fēi zhǔliú, non-mainstream/alternative).
culture
非主流 (fēi zhǔliú) was a Chinese youth subculture in the late 2000s, similar to emo/scene culture, known for dramatic hairstyles and heavy photo editing.