Aluminium foil is made a lot like pasta: you put an ingot in a mill, then rolls press it from both sides over and over so the ingot becomes thinner and lengthier. And at the final step the foil is so thin that you need to mill two layers of foil at the same time, otherwise it'll break. The side of the foil touching another layer of foil becomes dull. And the side touching the roll becomes shiny.

Note that for all practical purposes both sides behave the same.

Fun blog post.

In Australia we call aluminium foil 'alfoil', and have for 40 years or more.

Do other English-speaking countries not?

The shiny side prevents the alien voices from taking over your body. And the dull side prevents the government from monitoring you.