How to repair ships without going back to a shipyard in Stellaris?

There's only one way to repair ships without going back to a space port, you can research a tech gained from space beings that allows organic armor that repairs over time. From what I know it will repair slowly and not during combat but you don't need to go to a space port to repair. You can send your ships to vassal ports if you have any nearby but that's about it.

I forgot to mention it's a tech you need to apply to your ships, this can be done by researching the tech and then applying it to your ships via ship builder and then upgrading them at a port

With the 1.9 patch version there are four ways to regenerate your fleet's hull points away from space ports:

• Regenerative Hull: this is an auxiliary slot item that can be installed on your ships. This requires researching technology recovered from space monsters (amoebas, crystals, etc)

• Living Metal: this is a strategic resource found within the galaxy. This requires the Living Metal research, which is unlocked through a random event triggered by surveying the galaxy

• Nanobot Cloud: this is an aura slot item available for military bases

• Engineer: this is a trait available to the Admiral leader

These may change in 2.0 and Apocalypse (for example, the new Titan class ships will have aura slots as well)