Word for how the white space between words can form patterns

There’s a specific term for this, and even an entire Wikipedia article:

In typography, rivers (or rivers of white) are gaps in typesetting which appear to run through a paragraph of text due to a coincidental alignment of spaces.

They have this example:

Dan Bron's comment is exactly the features you're describing. You can consider this a typography example of negative space:

Negative space, in art, is the space around and between the subject(s) of an image. Negative space may be most evident when the space around a subject, not the subject itself, forms an interesting or artistically relevant shape, and such space occasionally is used to artistic effect as the "real" subject of an image.

You could also look up "visual poetry" in an image search which shows nice examples of negative space formatting being used purposefully. This is a poem by Jennifer Met: