The oldest known source of the ancestral story of creation is found in a poem called Völuspá, which can be found in the Poetic Edda. Jackson Crawford’s version, which I am using as a source, says Völuspá literally means ‘The Witches Prophecy.’ I’ve heard it translated as ‘The Seeress’s Prophecy’ and as ‘The Sybil’s Prophecy’. In any case, the story is told by a Völva (witch, soothsayer, or seer), to the God Odin when he beckons her to tell of the oldest deeds of Gods and men.
The witch recalls the giants that raised her in ancient days, of nine giantesses and the seed from which Yggdrasil sprang. In Völuspá stanza 3
It was at the very beginning,
it was Ymir’s time,
there was no sand, no sea,
no cooling waves,
She doesn’t say much more about Ginnungagap, but we can learn more from the Prose Edda in Gylfaginning where the kings of Valhall, High, Just-as-high, and Third, answer Gangleri’s questions about the earliest times.
Long before the earth was created, at the north and the south poles of the void called Gunnungagap, the world of Niflheim, with poisonous vapour and icy rime and the flaming impassable region of Muspell existed. From the energies of these two extremes Ymir appears:
there was a quickening from these flowing drops due to the power of the source of the heat, and became the form of a man, and he was given the name Ymir.
Ymir’s sweat and his legs create a race of frost giants:
there grew under his left arm a male and a female, and one of his legs begot a son with the other, and descendants came from them.
Next to rise from Ginnungagap is the primordial cow Audhumla:
when the rime dripped, was that here came into being from it a cow called Audhumla, and four rivers of milk flowed from its teats, and it fed Ymir.
Audhumla licks the salty rime-stones protruding into Ginnungagap and a man named Buri appears. Buri has a son called Bor. Bor marries Bestla, the daughter of a frost giant and they have three sons, Odin, Vili, and Ve. It is interesting to note here that the three kings of Valhall who are telling Gangleri this story, are possibly Odin himself, and perhaps his brothers? This question likely cannot be answered.
Bor’s sons go on to kill Ymir, believing him to be evil, and drown the entire race of frost giants with his blood, except for one, Bergelmir. All frost giants are descended from him.
Bor’s sons went on to make the earth from Ymir’s body, the sea from his blood, trees from his hair, the sky from his skull and the clouds from his brains. From his eyelashes they fortified an area and called it Midgard:
And from his eyelashes the joyous gods made Midgard for men’s sons, and from his brains were those cruel clouds all created.
They made the first man and woman from two logs found on the seashore, a man called Ask, and a woman called Embla, the ancestors of all humans.
So we know a little from these two sources about how our ancestors viewed the creation of the cosmos. The appearances of Niflheim and Muspell, Ymir and Audhumla are left unexplained, but the ice and the fire and the frost giants and the sustaining cow are at the core of everything in their worldview and indeed are an important basis for understanding all the stories that follow in the records left to us.
Völuspá in The Poetic Edda:Stories of the Norse Gods and Heroes by Jackson Crawford, Hackett, 2015
Gylfalginning in Snorri Sturluson: Edda, translated by Anthony Faulkes, Everyman, 1988