Well, the brightness of celestial objects is also sometimes negative:

> The apparent magnitude of known objects can range from −26.832 for our Sun to about +31.5 for objects in deep space imaged by the Hubble Space Telescope.[3]

See https://en.wikipedia.org/wiki/Apparent_magnitude

0db is usually defined as the loudest sound that the audio system can produce. Hence, everything else must be negative.

That is dB full scale where 0 is an absolute ceiling and you can deduct from there.