When seeking to dominate the galaxy through either trade, diplomacy, or good old-fashioned warfare, expanding one’s borders is always important. Given how complicated such a large scale strategy game such as Stellaris can be, it’s important to understand some of these core features when constructing your own empire. One of these is expanding your borders and colonizing various planets around the galaxy.
The sphere of influence will help dictate where and when you can build various space stations in a given area of space. You cannot actively send ships into another empire’s borders unless you have either secured a treaty with them or declared war upon them. Your best bet for expanding outwards without dealing with other aliens is through Frontier Outposts. These allow you to have larger expanded territories, so acquiring them early on should be your top priority. This is especially important since only a few races’ technologies will actually expand the borders, so you don’t want to be held up on waiting for them.
Now you don’t need to actually hold onto Frontier Outposts indefinitely, as they can be disbanded once your various worlds can sustain the borders alone. Think of these outposts as ways to mark off the area of your Stellaris faction without having actual planets to back this up. You can only construct outposts or colonies near your current border lines, so always try to expand out towards these areas. Remember, not every alien race can colonize every planet, so make sure to either enslave or uplift certain species so you can continue to colonize. The more outposts you have inside, the bigger your empire’s influence will grow.
Always be on the lookout for new outposts and you will be able to expand your colony without a need to even speak with other races. Diplomacy is overrated anyway.