tl;dr NPPDarkCrypt author hasn’t put it in the plugin manager.
The plugin manager is just a tool for convenience. If a plugin is in the manager doesn’t mean it is official (and visa versa). It is up to the plugin author if they want people to download it through the plugin manager or not. Plugins submitted to the plugin manager are roughly checked to make sure they don’t do anything obviously malicious but that doesn’t mean every line of code is reviewed.