Is there a part of the government that officially doesn't exist?

Answer 1

Not exactly. There are some things the government won't talk about in order to protect vital intelligence or lives, but there is no such thing as a "secret agency". The previous answer about Delta not existing is wrong. It does exist, however, the unit's name changes every few years to confuse foreign intelligence agencies, such as the Chinese and North Koreans. There are several books by former operators detailing their time within the Unit, one of them is by the Unit's founder himself, Colonel Charlie Beckwith. However, Delta's current name is classified in order to protect the lives of its operators and any ongoing operations they may be conducting. The last known name of the unit was the "Army Compartmented Element", but that has probably been changed already. Before that, it was called the "Combat Applications Group".

Another lesser known agency is the US Army's Intelligence Support Activity. While this unit also frequently changes names, previous ones including "Grey Fox" and "Cemetery Wind", their mission is to collect intelligence ahead of any operation carried out by Delta or DevGru.