。
She's afraid of the dark.
Literal
She [topic-は] darkness [subject-が] is-frightening.
暗闇 ('darkness, the pitch dark') is the noun for unlit space, distinct from the broader 闇 ('darkness') which often carries metaphorical weight (the underworld, hidden truth). 怖い ('frightening, scary') is an i-adjective of feeling — the source of the fear takes が, the experiencer is the topic. A simple frame for a common phobia.