Scientist have tested for the first time if antimatter behaves the same as matter in gravitational fields - it should and it did. The article is by one of the experimenters and involves some 'Big Science'.
Antimatter: we cracked how gravity affects it – here’s what it means for our understanding of the universe
It seems there isn’t a sci-fi part if the universe in which everything is made of antimatter.
theconversation.com