Then I noticed a small plate of complimentary marshmallows near Chloe’s elbow and it suddenly seemed clear that I didn’t love Chloe so much as marshmallow her. What it was about a marshmallow that should suddenly have accorded so perfectly with my feelings toward her, I will never know, but the word seemed to capture the essence of my amorous state with an accuracy that the word “love,” weary with overuse, simply could not aspire to. Even more inexplicably, when I took Chloe’s hand and told her that I had something very important to tell her, that I marshmallowed her, she seemed to understand perfectly, answering that it was the sweetest thing anyone had ever told her.
(Alain de Botton, On Love, p.80)
My mother tongue being quite different from English, reading the latter language's structure of the highlighted part is in fact more than working out a most difficult puzzle. This is what I understood: what is shifted from the position of complement of about and what clause is the complement of will never know. This would have remained no question if there was the that in front of a marchmallow. If then, that clause simply should have been seen as so-called a real subject with dummy-subject, it. Would you solve me the puzzle?