According to the poet Hesiod, the first power in the Universe was vast void named Chaos. The word Chaos is derived from the Greek chaino, "to gape, yawn". Chaos was the source of the first deities;
Contrasts with Christian Creation Stories
In the Biblical Creation story, the Earth was "without form, and void; and a darkness was on the face of the deep"1, and the Universe is created by God shaping this void. However, in Greek mythology, Chaos is the originating force.
Transition to Gods of Olympus
The greatest triumph of Zeus was seen to be the establishment of a firm rule over the primal, disorderly forces of Chaos. Greek mythology favours these Gods as they are seen to be more powerful and orderly.
1. Genesis, King James Version.