Create a space with the "Anonymous" group having the "view" permission. Disable ALL group permissions. When browsing anonymously, the space is accessible from the Dashboard. Once you have logged in, the space is no longer visible. Users which have logged in are not inheriting the permissions given to the Anonymous group. Also, when the Anonymous view permission is set globally, all spaces which are created should have the Anonymous view permissions by default. If a space is meant to be hidden in this instance, a space admin will have to set these permissions specifically. Carrying over the Anonymous view permission as a default if it is set globally will allow users to browse ALL content in Confluence without logging in, unless the space is PURPOSELY changed to remove the Anonymous view permission.