There are many restricted areas and some prohibited areas for flying in the US. most of them are used for military training or weapons development and are often 'cold' meaning you can fly into them. There is a prohibited area around the White House, for obvious reasons. There's a lot of restricted (not prohibited) area around Edwards Air Base in California.
There's also MOAs (Military Operations Areas) and training routes which are not restricted, but you have to keep an eye out for military jets. All these areas are shown on NOAAs Sectional and WAC charts, which you can buy at most small airports..
There's no secret about the existence of any of these areas, or where they are.