Before the big bang, there was nothingness. There was no empty space and certainly no matter in it. It was not as if there was an empty universe waiting to be filled with matter. Space itself did not exist; there was no 'place' for things to exist in. Then suddenly, an enormous explosion, far greater than anything that has occurred since, brought the universe and all the matter in it into existence. The matter was all crunched into the size of about an atom, but contained all the matter in the universe.

In a matter of an incredibly small portion of a second, the universe experience "inflation" and ballooned to the size of a golfball. That's the equivalent of a golfball increasing in size to that of the earth in a trillionth of a trillionth of a trillionth of a second. That's fast! As it did so, the mass of all that matter was spread out more and more as the universe continued to expand.

The particles in this "soup" of the early universe were mainly quarks and of course there was tons (not literally) of radiation and light energy. Since the matter of the entire universe was stuck in such a small space, it was extremely hot (much hotter than the center of the sun) and the particles were so energetic as a result that they wouldn't stick together to make protons, neutrons, and electrons http://www.protonneutronelectron.com.

That was fine because as the universe continued to expand very quickly, the heat spread out and eventually (after thousands of years) the quarks started to start sticking together and making the particles that we are familiar with today. These newly formed protons, neutrons and electrons eventually began combining into simple atoms such as hydrogen and helium. Over time, these atoms began to coalesce and form giant clouds of gas. The force of gravity caused the gas to start spinning. The gas was naturally pulled toward the center and over time, more and more gas started to crunch into the middle with an ever-increasing gravitational force.

After enough time passed, the mass at the center began to heat up because of the massive gravitational force. Once it got hot enough, the helium began to get so much energy, that helium started fusing to helium and the process of fusion began. This is when a star was born and the outer parts of the gas cloud also coalesced into planets. Thus was born our solar system and billions like them.