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Seetee by Jack Williamson?
- This's in a world where many asteroids in the solar system are formed of antimatter (here called "Contra-Terrane," "CT", or "Seetee"). Our characters hypothesize that came from an extrasolar antimatter rogue planet which entered the solar system in the distant past.
Not if the rest of the quote that I just added, from "The initial plan is" onwards, is accurate. Doesn't sound like The Slow Path is used in the Seetee series.
And it might be this same work from The Slow Path:
I'm fairly certain the two stories are unrelated, yet both stories share partial similarities with a lot of other works. For instance, Minus Planet is the first mention of Anti Matter, AND in the form of a Rogue Planet.
This request has gotten me skimming a lot of the Science Fiction Short Stories that are available to me. Mostly among those published by Analog Magazine, thinking that the two example are oddly familiar and that they seem to describe short form fiction. Part of a larger plan to make work pages for some of them, like Minus Planet.
Admittedly, the second story provides much ambiguity...
edit: minor correction
Edited by skewview AFK with issues, will return

Antimatter mentions this unknown work: