Not much, really. Antimatter is about at good as it gets with mass to energy conversion. A single pinhead of it could demolish a city with kilotons worth of tnt. The only reason I can think of would be to make it extremely difficult to produce in bulk, but that certainly won't prevent it.

@Crabford You say that starships are readily available to civilians. A starship is a spaceship that travels to the stars. Then you say that antimatter is only used in warp drive which only larger capital ships or specialized exploration ships would have. So how do the starships without antimatter or warp drive travel between stars?

@M.A.Golding For the purpose of this question, starship refers to any spaceship. The civilization ships are incapable of interstellar travel, but these ships can be transported inside motherships that do have a warp drive.

@hehe3301 Don't forget the mass of the antimatter is only 1/2 of the total matter converted to energy. Thus, for the antimatter m=0.02324g for 1kton TNT equivalent. I'm pointing this out because it's a common mistake to forget that you're also annihilating an equal amount of regular matter. I agree the base statement of a pinhead of antimatter demolishing a "whole city" is probably a bit of an exaggeration. If, or how much of an exaggeration depends on your definitions of "demolishing", "city", "pinhead", and the antimatter's density, etc. However, it's certainly enough to ruin your whole day.

Okay, it's a bit exaggerated. Maybe a box of pinheads would do the job, I didn't do the math, really. I was trying to get the point across that antimatter is a small and potentially compact method for a high destruction yield. It's hard not to weaponize the stuff when the quantity needed to kill thousands can easily fit in someone's wrist watch (so long as you can compact the containment method as well).

Note: all else aside, plasma-based projectiles are not viable.

If controlling during creation and delivery is difficult, then using it as a weapon is at least that much difficulty. But if it is a “common starship fuel,” then obviously those difficulties have been sorted.

@JohnLocke and others, please don't answer questions in the comments. It violates policy and bypasses quality control. Comments are for suggesting improvement, and asking clarifying questions to the OP

Just make it expensive. It only takes a little bit to power a ship.

@Aethenosity Thanks for the heads up. I thought that comments could be for half-answers, but that post makes sense. Next time I will post answers instead of giving half-answers in the comments.

Please note a similar question was asked on 23 July 2018. worldbuilding.stackexchange.com/questions/118966/…

What kind of anti matter is for sale? Like matter there is a whole range. And some of it would be harder to store (e.g. if only positrons are generated than storing those in a small device would be very hard due to their electric charge). A starship might solve this with a huge magnetic storage in space next to the ship. Basically the starship might need vacuum or the extreme cold to store it.

Note that there has been research in this (though I cannot find the PDF on my computer atm), and the proposed anti matter propulsion was anti hydrogen fires into a large vulome of regular hydrogen, and maginetically contained. Not using hydrogen means semi random matter being created when part of it are stripped away by anti matter, and thus you would have a mix of unstable matter in your drive. That includes free neutrons (and anti neutrons) which would be hard to contain leading to neutron embrittlement of the drive walls. So using liqued anti hydrogren might be best.

And that needs very cold temperatures to store which might only be practical in space. (20.28 K, 36,504°Ra, −252.87 °C, −423.17 °F) (This was meant to be a comment but got a tad long).

The problem might lie not even in the antimatter, but in the ships themselves. Any spaceship fast enough to go on interstellar journeys has enough kinetic energy to cause planetary extinction events if it crashes into a planet.

I argue that animatter is not a fuel. It's not something you transfer, store, mix, buy or sell. It requires a reactor and exists only in a reactor. It's not the same as radioactive material in nuclear reactors. Animatter does not want to exist. The reactor forces it to exist, and if the reactor stops doing that the result is a big explosion. You can't put animatter in a bottle and give it to someone else, because that bottle would look alot like a reactor. You can say reactors are common but you can't have them around populated areas. They wouldn't stay populated for long.

@Hennes the more highly charged the matter, the easier it would be to store.

Would it?

Why?

In my understanding. Say I have anti iron. Nice and stable.

But once I charge it the electric repulsion would push my fuel apart.

Not saying that you are wrong, but it does not compute to me