Emphasise your peace with big friendly vessels. Ensure it with big data. Fight your wars with hidden nanotech.

You need 3 basic vehicles:

• A 'peacekeepers' ship. Has:
  • Powerful Shields.
  • Ftl.
  • Self defence style weapons (close in railgun / intercepting laser beam weapons, etc).
  • No visible long range weaponry. (Some hidden long range missiles can exist - but the vessels appearance must appear friendly)
  • An ambassador, authorised to accept new planets into the empire, as well as make trade, peace treaties, etc on behalf of the empire.
  • A "press secretary" or "civilian liaison" or "propaganda sharer". Someone with no power but talks to the people and puts a friendly face on things as their job. Always in the media.
  • A military commander and a few underlings.
  • Support crew. Pilots, engineers, ships doctor, whatever is needed.
  • Carries hundreds of smaller "UAV" vehicles.
  • Scattered throughout the known galaxy. 1-2 ships per conquered solar system to "keep the peace". 3-4 ships per solar system on the empire's edges.
• "UAV" vehicles.
  • Small sphere. Size of a basketball. Anti-gravity drive. - Can have multiple variations if spheres are boring.
  • Has an AI core in it for tactical decision making.
  • Can get strategic direction from an underling on the peacekeeper ship
  • Small shield to make it difficult to attack.
  • No direct visible weapon.
  • Only thing visible on the surface is friendly messages: "Keeping you safe" sorta stuff.
  • Carries millions of nanobots, controlled by a link to the AI core.
• Nanobots do the bulk of your work.
  • Tiny small robot. ~1 nanometre size. Smaller than a grain of dust.
  • No local AI. controlled by the nearby UAV.
  • The UAV directs nanobots over everywhere within communication range - maybe 3-4 UAVs per continent, and a few UAVs in orbit.
  • Can get in anywhere and build up big data. They map the entire galaxy. They listen to every conversation. They record every facial expression, every printed word, etc. They get into computers and copy all the data. They get into brains and directly read thoughts. Data is sent to the UAV, to the ship, and back to some central processing facility.
  • Can sabotage enemy military equipment, decay critical bearings, crack fuel pipes, short circuit critical computer chips, etc.
  • Deals with enemy soldiers, military commanders, politicians wanting to not be in your empire, or journalists digging too deep by literally getting in their heads and: - Releasing positive hormones when the empire in mentioned to associate good things with cooperating with the empire. Releases negative hormones when fighting the empire is contemplated. - Same hormone play is used to socially isolate anyone opposing the empire from the rest of society. Those who oppose the empire are unable to make friendships or advance politically. - Deleting / blocking negative memories of the empire. - Blocking the skills you need to fight the empire. - Or if all else fails - inducing heart attacks, strokes, cancer, seizures, etc. - They can also block all memory of the nanobots, if you notice them in something and investigate you'll conveniently forget they are there.

Everyone in the empire has a wonderful life, and as the empire tries to expand, most people get a good feeling about joining and peacefully vote to do so. Some people oppose it but their opinion doesn't carry much weight, (and a few have died of natural causes).

Your empire occasionally comes across a war hungry people, but their weapons are always poorly designed and fail in the heat of battle.