This is one of the "Big Questions" in modern cosmology. I think it is fair to say that we don't know for certain. What is really troubling is that we may never really know because we can never re-create these initial conditions. All we can do is launch 'educated guesses' and conjectures based on where our best, or at least most interesting, theories seem to be leading us. We know that 'space' and 'matter' are two very different things, but long ago they were actually one and the same. Because Albert Einstein's general theory of relativity works so well, we have to accept for now its description of space and time as only aspects of the gravitational field of everything in the universe...gas...energy...matter...light. Near the Big Bang, gravity amplified itself by feeding off of its own energy in a complex, contorted, and brief state that ended when this gravitational energy produced the first generations of particles and anti-particles. So, matter was once part of space. Space remains indistinct from gravity, and so everything we see around us was once part of the invisible field we call gravity, which flashed into existence billions of years ago. Like a car rolling down hill, the momentum of this event is still with us and drives the expansion of the universe, and the clumping of matter into galaxies, stars, planets and ourselves!
