Now I'm confused. Is Antifa mainstream Left then? I think most people would disagree. What term is more appropriate to describe them? Genuine question.

Anarchists, socialists, and antifascists have been traditionally grouped under the umbrella of "the Left". It's odd seeing people try to push parts of the original Left out into the "Alt-Left", almost like gentrifying a political space.

(Antifascism itself dates back to early 20th century opposition action against fascist tides in Italy, Germany, and Spain; the comparison to the modern phenomenon of the Alt-Right feels historically inappropriate.)

> Is Antifa mainstream Left then?

It substantially overlaps with the mainstream of the left; note that in the US, the dominant (though decreasingly so) faction of the Democratic Party is center-right, not left at all.

What exactly is your question? You have about 3 layers of circular reasoning and undefined terms.

Many thought that the alt-right is not a real thing either. Turns out if you label something and it sticks, it becomes real.

“alt-right” is a label created by people to refer to themselves, and distinguishes their group from the establishment right.

“alt-left” is a label created by defenders of the alt-right to refer generically to it's opponents. They aren't really parallel terms.

I did ask :) It appears as if the Alt-Left is already a thing. Otherwise people would be asking for clarification on what I meant by it.