When I read manga, I sometimes stumble on speech bubbles like the one below. What does it mean? I have never seen something like this in other comics or graphic novels.
That's just usual dots like this "....". That's because originally, text in manga (which is in Japanese) is written vertically, like this:
Usually, I interpreted it as "The character have [something to say/opinion/thought/knowing some information] about the matters in hand, but choose not to say it yet". Well about why people that type it keep it vertical, that's their choice. And in the case with the screenshot you gave, IMHO typing it vertically looks better than typing it horizontally.