Before the beginning of time it is assumed that the only particles were anti-particles. Therefore time had not yet begun, but time had to start somewhere, so within the big bang theory there are many sub theories as to why these anti-matter particles became matter particles. One theory is that enough of the anti-matter was somehow combined that it became positive and would have automatically become matter. But I'm getting ahead of myself. Lets look at how we view the universe as the big bang shows us.
Lets look back into the past.
But not by starting at the beginning, but by working our way back to the beginning. It's well known that we can see stars and galaxies as they were in the past and even how the pollen in the air we breathe was from hundreds of years ago. By looking at the universe this way they work their way back to a fraction of a second after the beginning of time. And as we get closer the beginning the universe becomes hotter and denser. 13 billion years after the big bang is what you're looking at right now.