I'm not quite sure how eclipse was intended to work on opponents creatures when they are anti mattered, but the current way does not seem right to me. I believe that there may also be an issue with eclipse being played after the create has been antimattered, but i haven't tested it.
The following happens:
1. Opponent has create with Darkness on it
2. Player (me) plays antimatter on opponents creature, (say a 4/3 vampire, which is then turned into a -6/4 vampire).
3. Player (me) kills eclipse.
Result:
Creature loses 2 attack (so the -6/4 vampire turns into -8/4 vampire). This seems to make eclipse effect the creature twice, once when it is played, and once when it is destroyed.
In my opinion, it seems more reasonable for the creature to revert to its previous stats, with the attack still negative, which would in this example make the vampire revert to a -4/3 creature.
I think this is worth looking into.