In extremely small amounts – extremely, extremely, extremely small amounts – getting caught up at the Van Allen Belts. Still, as energy sources go it doesn’t get any more efficient than antimatter – besides, any information on how to stably store antimatter ‘naturally’ for any length of time would be handy when it comes time for us to do it artificially.
So I guess that we better write all of this down for the benefit of the next civilization that decides to have a manned space program, so that they can properly exploit this information. I hope that they like human rights!
PS: Nooooooo, I don’t think that I would quite trust private space development with antimatter quite yet, thanks.