So, humanity has invented an engine that while not capable of FTL travel, is still incredibly fast (IE cross the solar system in a matter of weeks or even days on certain ships). We make colonies and R&D facilities all the way out to Pluto. Trade and exploration have exploded in profitability, and mining asteroids have created the first trillionaires on Earth and its major colonies.
Now the only problem, how does one keep such a large area secure? Piracy would run rampant with how much spaceships have to cover. There aren't enough warships to cover the whole of the solar system. How do you prevent pirates from just raiding mining facilities and stealing from freighters and running away before any military ship can make it?
The force of pirates
Pirates basically have whatever ships they capture, with guns and missiles stuck on it. Their armor is poor, weapons are meh, rarely do they have an actual military ship, their engines are still just as fast. There are a lot of them, and because they have mostly civilian ships it can be hard to tell the difference between them and a civilian vessel at a distance.
force of the... civilized world
Heavy armor, insane weapons and missiles, not as fast, but if they catch a pirate, that pirate WILL be obliterated if they don't surrender. There isn't very many of them, so they've got a big job dealing with so many pirates spread out in such a large space... literally.