After Champagne yeast makes carbon dioxide inside a sealed bottle, why does much of that gas stay hidden in the wine until the cork is opened?

Answer: Pressure keeps CO2 dissolved

Alcohol creates bubbles — Alcohol changes aroma, body, and solvent behavior, but it is not the hidden spring that stores Champagne's fizz. The crucial condition is pressure: in a sealed bottle, CO2 is forced to remain dissolved in the wine. Once the cork leaves, pressure drops and the dissolved gas can finally expand into bubbles.

Pressure keeps CO2 dissolved ✓ — Right: the bottle becomes a pressurized CO2 container, often around 5-6 bar, roughly several times ordinary air pressure. Under pressure, more carbon dioxide can remain dissolved in liquid; when the bottle opens, that balance breaks. The fizz is less like air trapped in grape juice and more like a compressed gas reservoir suddenly given an exit.

Cork absorbs extra gas — Cork can exchange tiny amounts of gas over long aging, but it does not absorb a spare stash of fizz for later release. In fact, closures are chosen partly because losing CO2 through them is a problem, not the mechanism. The hidden gas stays mostly because bottle pressure keeps it dissolved in the wine.