Scientists generally believe that the universe was created from an event known as the Big Bang; however, the actual cause of the Big Bang remains unknown. The phrase "Big Bang" was coined by scientist Fred Hoyle.
Scientists believe that about 13 billion years ago all of the energy, space, time and matter in what eventually became the universe were compressed into a singularity, a tiny point. Some force caused this point to expand, and it has continued expanding ever since.
Within a nanosecond of the Big Bang, the universe was trillions of degrees Kelvin in temperature and hundreds of millions of miles across. The universe was too hot for atoms to form, but it was a soup of photons and particles with various types of properties. Some of these particles no longer exist today.
As the universe cooled, quarks started to bind together forming protons and neutrons. After a second, the protons and neutrons began to create the nuclei of hydrogen and other light elements such as helium. It took a few hundred thousand years for full atoms to form. At the same time, the universe became transparent as photons were allowed to stream everywhere.
The first galaxies probably came into being about 12 billion years ago.