DEX is the Android bytecode format used by apps on the Dalvik/ART runtime. A DEX file contains compiled instructions that Android can execute, and a single app may ship with one or many DEX files. Because the code is machine-readable by the platform, it is a normal part of app packaging, but it is also a useful hiding place for malicious logic.
In attacks, malware may load DEX code at runtime instead of exposing all of its behavior in the initial APK. This can make static analysis harder, delay detection, and let operators swap or activate features after installation. Defenders look for suspicious class loading, unpacking, reflection, and in-memory DEX loading, then combine that with runtime telemetry, permission review, and network monitoring. In mobile security, DEX is important because it sits at the boundary between legitimate app execution and code that appears only when the malware decides to run it.