For every one billion particles of antimatter there were one billion and one particles of matter. And when the mutual annihilation was complete, one billionth remained - and that's our present universe.
To regard one's immortality as an exchange of matter is as strange as predicting the future of a violin case once the expensive violin it held has broken and lost its worth.
The question of the origin of the matter in the universe is no longer thought to be beyond the range of science - everything can be created from nothing. It is fair to say that the universe is the ultimate free lunch.
Hope for the best, be prepared for the worse. Life is shocking, but you must never appear to be shocked. For no matter how bad it is it could be worse and no matter how good it is it could be better.