Why do people refer Anakin Skywalker as "Darth Vadar"? ?

I understand the name Darth Vadar made up by the Sith, but in reality,
that's not his real name. Like, in episode 6 when Luke fights his
father, the battle is really called "Anakin skywalker vs Luke
skywalker" The two most powerful jedi's in the universe, and the two
skywalker's face off one final time. Everyone still refers to him as
his made up name instead of his actual real name for some reason. :/