Humans will probably destroy each other before any natural disaster strikes. -___-
Those who survive will be left to fend for themselves. Imagine being able to have enough anti-matter to vaporize a continent. Whether an impact of this size happens naturally or not, nothing will be left that can be consumed on earth. Most life will become extinct due to poisonous radiation. Radiation from antimatter has not been tested..well, we can’t. If radiation doesn’t kill you, starvation and thirst will.