Apart from the World Wide Web we’re familiar with, there is also something called Usenet, which consists of ‘newsgroups’ available in a different format. Newsgroups are devoted to specific interests or topics, and are either moderated or unmoderated.
The unmoderated newsgroups begin with the prefix ‘alt.’, so ‘alt.right’ is used to mean those on the right side of the political spectrum who do not follow the leadership of established conservative thinkers, thus ‘unmoderated’.
Thanks for that etymology.
That explains the root of the origin, but not how the Left and the Media intend it to be used, IMO.