The Combine

THE RULERS OF THE GALAXY

The Combine is a (loose) organization that controls the galaxy. They don't interact with the common people, but their presence is known and felt. 

The Combine is made up of the elites from each Monolith, the leaders of each planet. They're in an indefinite peace treaty, though they use loopholes to get around this, using underhanded methods to get back at each other.

Saying 'The Combine' could equally refer to the collection of planets or the people behind it. It's a vague concept