Interjections are words or phrases used to convey surprise, as well as emotions such as joy, shock, excitement, fear… the list truly goes on and on.
An interjection can be followed by an exclamation point, question mark, period, or comma. It all depends on whether it stands on its own or if it's part of a sentence—and how dramatic you want to sound.
Examples of interjections include:
Yeah, they look kind of sad without exclamation points. But, hey, maybe they are sad. You don't know their lives.