Rashi on the first verse of the Torah goes to great lengths to prove your very point: that the Torah does not intend to reveal the chronology of creation, but rather to describe certain dimensions of the creation. He masterfully brings many verses throughout Torah to support this usage of "Bereishet bara". In addition, he suggests a simple logical proof as well: If the Torah's intention is to chronicle the creation of first heaven then earth, where does water fit in, which is mentioned later both as a component of heaven and appearing before earth?
So what existed before Creation?
On one level, the answer is simple: God. However, this is also one of the deepest topics discussed in Kabbalah. Referred to as tzimtzum (constriction, withdrawal), it describes the inner dimension of what transpired between what you call "pre-existence" and creation. I'll paraphrase this idea as described by the Arizal:
Before all things were created, the Supernal Light was complete and perfect. It filled all existence. Since everything was filled with the simple Light of the Infinite, there was no empty space that could be considered space, emptiness or void. There was no concept of beginning or end. All was one unified, undifferentiated Infinite Light.
When it arose in His Will to create all the spiritual and physical worlds, He withdrew His Infinite Essence (1) from His Infinite Light (2), resulting in what would become the center point of the space about to be created. He then withdrew His Essence even further, to the extremities around this center, leaving a Vacated Space (3) in the midst of the Infinite Light, for all that was to be emanated, created, formed and completed (4).
He then drew a single, straight line (5) down from His Infinite Light into the Vacated Space. The upper extremity of this Ray was connected with the Infinite Light surrounding the Space, while its lower extremity descended toward the center of the Vacated Space. It was through this Ray, serving as a conduit for the Infinite Light, that Divine energy was drawn down to create and sustain the universes.
The Arizal goes on to explain that the Ten Sefirot are located in this Ray of Divine Light emanating down from the Infinite into creation. In this way, the array of the Sefirot is like a prism refracting the simple, undifferentiated Divine Light into the components necessary to generate the multiplicity of creation.
This process of tzimtzum is represented in the four-letter name of God, through which He is made known in the world. The letter "yod", which hovers above and points in all directions, corresponds to the Infinite Light. The first "Heh", which defines within itself a space differentiated from the "yod", corresponds to the boundaries of the withdrawn Infinite Light and the resulting Vacated Space. The letter "Vav", comprised of a straight line and which literally means 'connector', corresponds to the Ray of Light flowing straight down from the Infinite toward what would become the center point of creation. The last "Heh" represents the space expanded within that center point in which all spiritual and physical worlds were created.
Sources:
- Etz Chaim, Drush Igulim v'Yosher 1:2
- (1) Ein Sof
- (2) Or Ein Sof
- (3) Chalal Panui
- (4) Atzilut, Beriyah, Yetzirah, Asiyah, respectively
- (5) Kav
