What does Finns det ingen mat? - Jo, det gör det. mean?

See a translation

You can say "Jo, det finns." (Yes, on the contrary, there is.) When we say "det gör det", one "det" means the subject "it" (the food) and the other means the object "that" (existing: the thing it does). You can think of it like "it does that". In English, we can just say "it does".

"Does no food exist?!"
"Yes, on the contrary, it does."

This sounds strange in English, but it might help you to understand how to say it in Swedish. In Swedish, we can't just say "det gör" ("it does"), we must say "det gör det" ("it does that").

Even more confusing, we don't know which "det" is which: "det gör det" can mean "it does that" or "that it does". The second version sounds more normal in English, and shows that English and Swedish have common roots.