Could it be that…? (莫非)

莫非 (mòfēi) introduces a troubled conjecture about the likely cause of something puzzling: «could it be that…?, is it possible…?». The speaker already leans toward that explanation and floats it as a question. It keeps the literary reading 莫 (mò, «not») + 非 (fēi, «is not»), hence the double negative «isn't it that not…?» → «surely yes». It often closes with 不成 to reinforce the tone. The variant 莫不是 works the same and is folded in here.