What does it mean when Darry tells Ponyboy that he is living in a vacuum?What do they mean by living in a vacuum

What Darry means by this is that Pony is ignoring the realities of his life.  He is acting as if there is nothing going on in his life, like it is a vacuum that is empty except for the deaths of Johnny and Dally.

Darry is telling Pony that he has to quit acting like this -- he has to realize that there are all kinds of other things going on that are important.  He has to realize, especially, that school is important and that he could make something of himself if he really starts to apply himself to school.