There was no before the Big Bang—the Big Bang created both time and space as we know it.
At the moment of the Big Bang, everything in the physical universe expanded from a singularity: a point of infinite heat and infinite density. It is important to remember that we cannot imagine the Big Bang as a normal explosion, occurring at a single point in space. Rather, space itself expanded. By the same token, we cannot think of the Big Bang as occurring at a single point in time. Space did not expand in time. Space expanded with time.
This conceptualization of the universe’s birth is made possible by Einstein’s theory of general relativity. According to relativity, space and time are stitched together in a four-dimensional continuum called “spacetime.” Moreover, the fabric of spacetime is not merely a flat stage on which objects and processes in the physical universe interact. Spacetime is warped by the presence of mass, creating gravitational fields. Additionally, the curvature of spacetime governs the motions of particles, including photons, even though they’re massless. As the renowned 20th-century physicist John Wheeler quipped: matter tells spacetime how to curve; spacetime tells matter how to move.
By perceiving spacetime as an active part of the physical universe, we can more easily imagine how both space and time came into existence at the moment of the Big Bang, along with everything else we observe in the universe.
Further Reading: "Nothing: Surprising Insights Everywhere from Zero to Oblivion."