i occasionally send people the following message, on text chat platforms:
i've been asked what it means often enough that at this point i'm making a short blogpost.
"." is the empty message. it conveys whatever it means to say nothing, but explicitely — whereas a lack of message at all is not as intentional, and might result from me being for example away from the computer, "." just makes it clear that my reaction is to say nothing.
imagine a conversation in meatspace. you say something, and i react by just standing there, not saying anything. what does that mean? it can mean all sorts of things, depending on context! that's it, that's "." .