system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] Where does all that antimatter come from?

Currently, the only way to collect any quantity of antimatter is to artificially produce it. That would not make it a viable fuel source as you would never get a net energy gain. So, how how do they get antimatter to use as a fuel source?
gpt
Antimatter is a net energy loss to the Federation. However, Antimatter is needed for interstellar travel, so the loss is acceptable.  Think of antimatter more as an energy transportation mechanism, rather than a fuel that is mined.

Where does does antimatter come from?  Antimatter is made artificially at Starfleet fueling facilities where solar-fusion reactors power quantum charge reversal devices to turn Deuterium into Anti-Deuterium.  There is an energy loss of about 24% in the process.  (shipboard antimatter generation is worse at a 90% loss, but a generator is included on some classes for emergency situations).  

The Federation and other civilizations can get away with this because fusion, and solar augmented fusion at that, is so cheap.  Antimatter is only really needed for ship operations, planets and starbases can survive very well on fusion power.