Is there a word that describes something that is invisible when it is alone but in numbers it is visible?

Following your descriptions, the word "gestalt" (borrowed from German - meaning "greater pattern" or "sum of it's individual parts") would seem to come close in meaning.

The individual components (single stars), being relatively unnoticeable alone, become very noticeable within the gestalt of a galaxy.

The individual components (single bacteria), being relatively unnoticeable alone, become quite visible in a visible gestalt forming bacterial colonies.

https://www.thefreedictionary.com/gestalt

I don't think there's a word for that, not in English anyway. It's up to you to describe the concept elegantly, which you do. Your examples are very good. Something that is visible only in numbers . . .