It's not ruled out by evidence, AFAIK, and could explain some observational puzzles.

https://indico.fnal.gov/event/6199/contributions/94686/attac...

(that was 2013; perhaps observations since have made it less likely)

2021 paper: https://arxiv.org/abs/2105.08719

2020 paper: https://www.researchgate.net/publication/342573954_Antimatte... ("a large region of the parameter space remains unconstrained, most notably for nuclear-dense objects.")