I've played hours Dark Matter Dice, submitted my score a few times, but I haven't unlocked the challenger for the battledome. Is this challenger retired? a bug? or just rotten horrible luck?

I have also tried refreshing the main game page, no luck.

The challenger appears randomly while you play Dark Matter Dice. So you gotta play quite a bit and, in any given roll, it can appear.

You don't have to play to unlock the challenger, just keep refreshing the main page of the game, I got him recently this way.