It's very very very very hard to forge money convincingly enough to fool central banks. It's basically impossible for anyone but nation-state actors, and the only known cases date back decades. Today, it's even harder, because even if central banks were unable to identify forgeries, duplicate serial numbers or ones that have never been issued would be detected very quickly.

Even making forgeries that can fool normal automated tests, like those done in an ATM, is incredibly hard. It's certainly possible, just way too expensive.