From the interior of Sardinia to central Germany and Ireland, there is a lot of land that is uncolonized and seemingly empty on the political view.
Colonization of Uncolonized Land
As long as you have an adjacent city of at least 10 pops you can send one of these pops into an uncolonized location and this pop will claim it for their home country at the same cost as the one you pay for moving pops inside your own lands.
This will turn the uncolonized location to a city under your control, but it will not change the culture, religion or type of any of the pre-existing pops.
If you are a country with a high civilization level the local Civilization rating will now start to climb towards your country value. Making it more suitable for your freemen and citizens, but most likely making the original population of tribesmen quite unhappy.