The vast Universe was once smaller than an atom. This tightly packed particle exploded, and from it came all the material of which the Universe is made.
Once the Big Bang occurred, the Universe began stretching like the rubber skin of a huge balloon. Space and time began to expand at an incredible rate. This phenomenon is called the inflation of space. At the end of the rapid inflation, the Universe became cooler,and the particles that make up matter began to form out of some of the energy.
The gas and dust formed after the explosion, condensed into galaxies, that are speeding away from each other.