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 ...