What is the largest group in South Korea?

Could you please enlighten me on the matter of demographics in South Korea? Specifically, I'm curious to know about the largest ethnic or cultural group residing in the country. Is there a particular group that dominates the population, perhaps due to historical, cultural, or other factors? Understanding the composition of the largest group would greatly assist me in grasping the social and cultural landscape of South Korea. Thank you in advance for your assistance.