、。
The light's on, so they can't have gone out.
Literal
Light [subject-が] on-is [explanatory-のだから], they [subject-が] go-out-did [expectation-はず] not-exist.
Combines のだから (explanatory 'because') with はずがない ('cannot be that,' literally 'there is no expectation that'). The はずがない pattern is used to rule out a possibility based on evidence or reasoning. A detective-style deductive statement: 'evidence says X, therefore Y is impossible.'