Which sense do bats mostly use to hunt in the dark?

Hearing. A bat shouts and listens to the echo, and the picture it builds is fine enough to catch a moth in total darkness or to thread through a cave full of thousands of other bats doing the same thing. The calls are loud - up to about 120 decibels at the source, roughly a smoke alarm at close range - and mostly above human hearing, which is the only reason a cave at dusk is not deafening.

The bat has to solve a hard engineering problem: it is shouting and listening at the same time, and its own call is millions of times louder than the returning echo. It manages by contracting a muscle in the middle ear to disconnect its own hearing for the instant it calls, and relaxing it in time for the echo - dozens of times a second.

The blind spot in the phrase is the assumption that bats are blind. None are. Many see perfectly well and use vision for long-distance navigation, and the big fruit bats mostly do not echolocate at all, finding food by sight and smell. Meanwhile some moths have evolved to answer back: a few species emit clicks that jam the bat's returning signal, which is the only known case of an animal using electronic-warfare tactics without electronics.