。
Is that really true?
Literal
Truly so [conjecture-だろう]-[question-か].
だろう is the plain-form tentative/conjectural ('probably, I suppose'), here followed by the question particle か to form the slightly rhetorical/doubting 'I wonder if really so.' Together だろうか has a musing, uncertain feel — the speaker is probing rather than asserting. Common in internal monologue or when thinking aloud.