Since you aren't able to colonize, you must have chosen either "Pirate" or "Nomad" instead of "Normal" as your race trait.
In order to keep up with research, Steal&Analyse Ships, trade techs, and capture research planets.
If you can manage to con an AI into a trade alliance, you'll be rolling in money (relatively), and will be free to use those funds to build and maintain a very nice force to attack your ally's enemy with.
Don't ever expect to keep up with the number of ships they build.
The trick: Don't fight harder, fight smarter!
Make lots of hidden SY bases in storms and nebulae, perfect for refueling & repairing your ships, and making for great places to haul your plundered booty for scrap on analysis.
Always enter combat with an unfair advantage.
- Pounce unescorted ships with a large fleet in order to minimize your casualties and maximize the chance of capturing an intact ship.
- Always use tactical combat
- Sneak in and lay small mines in the path of enemy ships. Just enough to blow out their shields and weapons for an easy capture, but not enough to kill them.
Using the specialty mines (Ion, Null space, Tachyon, etc) can really help here.
Design your fleets and develop tactics as you would an RPG adventuring party.
A healer, a stealthy armored theif, a solidly shielded close combat Warship, and a long-range missileboat/archer.
Lay traps in the enemy's way, and ambush them on advantageous territory. (Shield depleting sectors and systems are excellent for capturing stuff
)