Yes and no. Anti-matter is something, yes. But the absence of something cannot also be something. It may (or may not) be something else, but being a shadow does not tell us what it is, only what it is not.
One remembers this from one's childhood:
"That that is, is; that that is not, is not" and the corollary: "That that is is is that that that that is not is not".
Ponder ye this:
In baseball (where a lack of runs scored is commonly called "nothing") each team takes turns trying to score before it commits three outs, with the visitor team getting first chance. Hypothetical game: the visiting team takes its turn in its half of the first inning and scores two runs before committing three outs. Now it's time for the home team to take its turn.
At this moment in time, the score is not "two to nothing" or two to zero; the score is "visitors two, home team coming to bat", i.e. the score is incomplete since the home team has not had a chance to score, therefore their score cannot be called "zero" or "nothing". Rather, their score does not yet exist. Which is distinct from "nothing".