When one of your fleets enters a system controlled by a faction you haven't met before, a first contact event triggers. This establishes initial diplomatic relations.
Initial Relations
Each faction has a natural disposition that influences how they view you from the start. Your proximity, relative power, and shared borders also affect initial relations.
After Contact
Once contact is established, the faction appears in your Diplomacy Panel. You can begin proposing treaties, monitoring their expansion, and adjusting your strategy accordingly.