An expanse within
Left to itself, the new "water universe" was a competitive community of electrons, nuclea and photons with gravity overpowering the other forces. If God had not spoken again, His work so far would have collapsed in on itself.
But once God starts something, He doesn't stop. He speaks again, beginning another day of creation. This time, he calls on the majority of the universal waters to be spread out (Heb: רָקַע raqa`). The noun form in the text is רָקִ֖יעַ (raqi`a), an expanding wall of water coming from deep within (תָּוֶךְ tevek) the sphere.
God proves to be several steps ahead of nature from the very beginning. As the water is agitated, perhaps by microwaves of compressed light, it rises through the mass toward the voice that called it.