This is my favourite piece of Duolingo wisdom thus far. Very sage!
Why is "sự tồn tại" being used? "sự" makes a noun but here the prase is used as the verb "to exist".
You've misunderstood. "Sự" here is part of "thật sự" which means "really", not part of "sự tồn tại".
Why not accept "doesn't"?? Come on guys fix it please
If you haven't already, please report such an error