I don't know whether or not anyone has tried it. It would appear as though the way you'd change it would be to open up GalCiv3MapDefs, find the line that reads <BlackholeResoure>Antimatter</BlackholeResource> and change it to <NebulaResource>Antimatter</NebulaResource>, though I'm not sure how well the game handles having multiple resources with the same spawn location.

Nope, didn't like it at all when I tried to change <BlackHoleResource>Antimatter</BlackHoleResource> into <AsteroidResource>Antimatter</AsteroidResource>. Gives me an error:

 

F:\Users\SNIP\Documents\My Games\GalCiv3\Mods\Custom Settings\Game\GalCiv3MapDefs.xml(90,15): error: element 'AsteroidResource' is not allowed for content model '(PlanetLikelihoodInAsteroidLane,BinaryStarLikelihood,PlanetTraitLikelihood,StormcloudLikelihood,StarsystemReplacedByBlackholeLikelihood,NebulaPlacementBuffer,DefaultPlanetClassSelect+,MapEditorPlanetClassSelect+,StarSystemWeightMin,StarSystemWeightMax,BlackHoleWeightMin,BlackHoleWeightMax,NebulaWeightMin,NebulaWeightMax,AnomalyWeightMin,AnomalyWeightMax,MinNebula,MinBlackHole,MinAnomaly,DeadWorldResource,GasGiantResource,AsteroidResource,BlackholeResource,NebulaResource,StarSystemTemplateGroup,DefaultBlackhole,DefaultRift,DefaultNebulaGroup,DefaultStormGroup,PlayerStartSpacingWideMod,PlayerStartSpacingNarrowMod,BlackholeSpacingMod,AscensionGateSpacingMod,RelicSpacingMod,PirateBaseSpacingMod,PlayerStartToPirateMinRadius,ReduceStarSystemBoundarySize,PirateBaseTightenRadius)'

So I copied the schema file over into my mods folder and changed BlackHoleResource into AsteroidResource so there was two of them. That gave me a different error as it first tried to load the default GalCiv3MapDefs.XML file first and found a BlackHoleResource and saw it wasn't declared. Error given:

 

F:\Program Files (x86)\Steam\steamapps\common\Galactic Civilizations III/Data/Game\GalCiv3MapDefs.xml(73,22): error:
no declaration found for element 'BlackholeResource'

F:\Program Files (x86)\Steam\steamapps\common\Galactic Civilizations III/Data/Game\GalCiv3MapDefs.xml(90,15): error:
element 'BlackholeResource' is not allowed for content model '(PlanetLikelihoodInAsteroidLane,BinaryStarLikelihood,PlanetTraitLikelihood,StormcloudLikelihood,StarsystemReplacedByBlackholeLikelihood,NebulaPlacementBuffer,DefaultPlanetClassSelect+,MapEditorPlanetClassSelect+,StarSystemWeightMin,StarSystemWeightMax,BlackHoleWeightMin,BlackHoleWeightMax,NebulaWeightMin,NebulaWeightMax,AnomalyWeightMin,AnomalyWeightMax,MinNebula,MinBlackHole,MinAnomaly,DeadWorldResource,GasGiantResource,AsteroidResource,AsteroidResource,NebulaResource,StarSystemTemplateGroup,DefaultBlackhole,DefaultRift,DefaultNebulaGroup,DefaultStormGroup,PlayerStartSpacingWideMod,PlayerStartSpacingNarrowMod,BlackholeSpacingMod,AscensionGateSpacingMod,RelicSpacingMod,PirateBaseSpacingMod,PlayerStartToPirateMinRadius,ReduceStarSystemBoundarySize,PirateBaseTightenRadius)'

So now I am going to alter the original GalCiv3MapDefs file as well and see what happens. . . .  So that works for stopping the errors, however the game doesn't spawn antimatter in the asteroid belts, just continues to dump obscene amount of durantium everywhere like it always does.

Going to try doubling down the asteroid resource by doing:

1) removed the BlackHoleResource line completely from all 3 files

2) added to the AsteroidResource line <AsteroidResource>DurantiumResource, AntimatterResource</AsteroidResource>

And that gives me.... well it sure didn't like that. Game started fine but crashed the moment I tried to make a new random game. Changed all files back to original configuration. I will probably just replace Promethion with Antimatter as I make no use of Promethion what so ever. That or have to either buy the map editor DLC or change game to permit good quantity of black holes to spawn.