Aside from being the thing at the end of a sentence, "." is sometimes used in internet slang to show you are annoyed/frustrated/done with the goings on of the situation or person/people around you.
Cameron: "Not gonna lie, Fortnite is pretty great"
Jack: "."
The . is used to express being emotionless.
Nikola : How are you?
Ahmad : .
When you are speechless, and have to say something online, or if you just want to start a sentence without "hi!" because that's too cheesy
Person A: I just ate my dog!
Person B: .