I'll clarify this.
At the beginning of time, there are two tribes of human beings. Grongi, and Linto. They have been at each other's throat for a long time until one day, Grongi's found a miraculous stone and used it's power to the extent that they were no longer human, but monsters and slaughtered the Linto tribes. Kuuga, a normal human being, asked the Overlord of Darkness for the same power. He grants him the same type of power and Kuuga uses this power to seal away all the Grongis. The Overlord of Darkness asks Kuuga returns a favor and destroys Agitos, evolved human beings that are not of his creation, but rather from the creation of the Overlord of Light who created the Seed of Agito and planted it in the humans who are the children of The Overlord of Darkness.
Kuuga refuses, thus sealing himself away in a tomb as we see in the very first episode.
In a way, Kuuga can be seen as the power of Darkness, while Agito can be seen as the power of Light. They coexists without having to face each other and still keep a balance.