The density is the amount of colonists per square kilometer.

Density is largely determined by urban development, with more urbanization allowing for higher density. However, rich colonists will try to acquire larger more luxurious properties, due to which high wealth can reduce density.

The land requirement for industry is also significant. When you build new structures, you will acquire the land for them, which is zoned according to the building type. I.e. building farms will greatly increase the area zoned for industry.

Density has an impact on growth, neither rural nor overcrowded places are particularly popular. Studies of some of the universe' largest colonies have revealed that most of them have a density of around 5000 citizens / km².