Dominant

This should end up happening naturally as your empire expands - you'll typically have cultural dominance over the planets that you have colonies on so as your empire grows you'll end up with the 15 needed for this.

You can check cultural status in the planet info window (hold Alt to expand), and it'll show the % dominance. If you find this gets eroded on your planets that are close to other empires then build more media/civilian structures to help increase culture and influence.