Joan Didion Essay

Although I was only able to read about 3 pages of Didion's essay, titled "On Morality", I believe I got the "gist", or what I think it's to be, of the story.

Here's a quote from the essay, "Particularly  out here tonight, in this country so ominous and terrible that to live in is to live with antimatter, it is difficult to believe that "the good" is a knowable quantity."

What does the author mean by this? I'm not quite sure, which is why I chose this quote out of the limited number of sentences Google Books allowed me to read. What does she mean by the term antimatter? Does she mean it in a literal sense, as in that tiny little speck of stuff like in Angels and Demons that can wipe out a city in a fraction of a second? For the second part of the sentence, "it is difficult to believe that "the good" is a knowable quantity.": meaning, it is hard to define "good" in terms of a definitive amount?

Hmm..