There are actually dashes, en dashes, and em dashes! But that's probably a piece of its own lol. I hadn't heard that before about the mirror dash! And I've gotta say that you've got the superior name. Visually, the image that conjures for me is a literal dash beside a mirror doubling in length, like how when you put your finger up to a mirror it appears to become two fingers touching at the point. I hadn't heard that before about it being wide as an M, but that makes sense then why the en dash is called that, too!

Thanks for reading, Annie!