Junk. Old and abandoned satellites may be a good source of fuel, shield or ammunition.
Mad
robo-miners. Sometimes, cosmic radiations may erase robot programs.
This kind of automatic machines, which are used by colonists to extract
minerals from asteroids to build space station, are almost armless. You
can recover some bonuses from them.
Human race is not the only species willing to expand in the galaxy. Aliens are always hostile and often well -armed.
Some sectors are guarded by mother ships. Always be aware of them.