This is why homebrewers buy hydrometers. It is the only way to tell if a ferment is done.
Half of the fermentable sugar in your wort becomes CO2, the other half becomes alcohol. As mentioned, some of it stays in solution.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk