No! A set of bits exists on disk. A piece of software (git) interprets those bits as a branch. The idea of a branch exists in the heads of the users - subject to the usual human limitations. While quite a lot of users will be able to be precise about what a branch means in terms of git blobs, there will be people using the tool and talking about branches who don't know that. But an idea, whatever it is, exists only in the way that people think and talk about it.
Even in this thread we have people fighting about what it is: https://news.ycombinator.com/item?id=38999486