Quantum theory describes electromagnetic and strong and weak nuclear forces.
Gravity, the fourth fundamental force, is described by the theory of general relativity.
Science lacks a unified, quantum gravity description for all four forces. According to our best estimate, the universe was born 13.7 billion years ago. The Planck epoch (before 10-43 second) is beyond our current understanding. Since Quantum mechanics and and general relativity break down behind this frontier, a robust description of the Big Bang theory can only begin afterwards. During the Planck epoch a unified, symmetry of of all four forces is assumed. At some point the much weaker gravitational force somehow breaks this symmetry and becomes separated from the remaining three, much more perfectly compatable, forces.
Distinguished astrophysicist, Joseph Silk (1994) writes that:
The Universe began at time zero in a state of infinite density. At least the existence of such a singular state is the expectation from extrapolating the present universe back in time. Of course, the phrase "a state of infinite density" is completely unacceptable as a physical description of the universe , infinities being abhorrent to physicists. An infinitely dense universe would be what is called a "singularity," where the laws of physics, and even space and time break down
The resolution of the paradox simply is that our theory of gravitation has broken down before reaching this extreme state. The first 10-43 second is inaccesible to our current theories..."
Silk (1994: 86) is clear that the big bang theory has replaced the creation myths of our ancestors, but he is careful to delegate the actual creation of space and time to "the pundits of metaphysics, theology and even quantum gravity." He writes:
It is difficult to separate fact from fiction, and faith from fantasy, as any conclusions are far removed from the domain of experimental, testable, or even fully consistent physics. A more presssing and relevant issue is the creation of matter. Early-universe cosmology squarely tackles the creation issue by formulating a physical description for the origin of matter , the very stuff that we, the earth and the stars are made of.
Silk, Joseph. (1994) A Short History of the Universe. Scientific American Library, New York.