Sent to the New Mexican.
Sacha Pyle asserts the "alt-left" is a fictional designation. I suspect by alt-left, people are referring to the motley crew of social justice warriors who have christened themselves "antifa", dressed out in battle gear and black, and who have been demonstrating, violently, during gatherings of the alt-right (added later: and against centrists and fellow leftists, even). These leftist warriors far more resemble Mussolini's blackshirted squadristi than anything I recall from the Civil Rights era of the last century, where peaceful tactics not resembling paramilitary assaults gained important ground, defeating violent adversaries by using the tactics of King and Gandhi.
The antifa should look at history. In the 1920’s-30’s era when left and right wing mobs fought in the streets of Germany, Italy, and Spain, things did not go well for social justice.
Finally, if one is going to ape one's opponent's bad behavior, there is a good chance one will be tagged with a similar moniker. Hence alt-left.
"...calling radicals the “alt-left” is mischievous, tarring those fanatics with their ideological rivals’ brush. But as Communists and Fascists showed, the political world is round. If you go too far left or right, you meet in the anti-democratic land of intolerance and violence." -Gil Troy, Professor of History, McGill University, in the Time article linked above.