This is one of the "Big Questions" in modern cosmology. The best answer we have right now reads very much like a tour of the entire field of theoretical physics and observational cosmology and astrophysics combined. No one ever said that the answer had to be a simple one...and it really isn't.

We know that 'space' and 'gas' are two very different things, but long ago they were actually one and the same. Because 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 and brief state, which ended in this gravitational energy producing the first generations of particles and anti-particles. These later decayed into, not only the familiar electrons and quarks, but also into the particles of light and the essences of the other fundamental forces in existence today. Mathematically we can describe much of this transformation, because many of its key ingredients have been seen by physicists already, at their laboratories. But the earliest conditions have yet to be artificially re-created so that we can thoroughly test our best theories.

So, gas 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!

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