Antimatter is matter going backwards through time.
No it isn't. Whilst that idea might appear to have some pedigree, (see retrocausality on Wikipedia), it's bunk I'm afraid. Antimatter is like matter, but it has the opposite chirality. Google on positron chirality. Whilst one can mathematically model the positron as a "time reversed electron", it isn't actually going backwards through time. It's more like it has the opposite spin. And charge. Have a look at the picture of a spinor here. It's essentially a Möbius strip, which is chiral. It has a handedness. If you go round the strip one way, it has a clockwise rotation. That's like matter. If you go round it backwards, it has an anticlockwise rotation. That's like antimatter. There is a sense wherein antimatter is "matter going backwards", but not through time.
From a matter-based observer does antimatter:
•Increase in entropy (and therefore decrease in entropy in its own time)
•Decrease in entropy (and therefore increase in entropy in its own time(
It increases, just like matter. Like what Alex said.
Option 2 would seem to explain why we don't see much anti-matter (it all went into energy).
It doesn't I'm afraid.