I see many times in comment sections of Internet forums such comments as the following:
A: This girl is so ugly and I know nobody will date her ever.
What does it mean?
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"That!" is an affirmation by the person saying it approving of what another speaker has just said.
Here are some similar ways of saying the same thing with perhaps different flavors from different parts of the US(or GB for 'spot on')
"you said it man" , "amen brother", "spot on", "absolutely"
It is also a type of interjection that even our more elite, intellectual friends might make saying "indubitably" : )