How did the universe begin?

Humankind has always puzzled over the origins of the universe. Scientists believe that the universe began with the Big Bang, a cosmic explosion that occurred between 10 and 20 billion years ago and threw matter in all directions. The universe began as a dense, hot fireball, a scrambling of space and time. Within the first second after the bang, gravity came into being. The universe expanded rapidly and became flooded with subatomic particles that slammed into one another, forming protons and neutrons. Three minutes later, when the temperature was 500 billion degrees Fahrenheit (280 billion degrees Celsius), protons and neutrons formed the simplest elements, including hydrogen, helium, and lithium.

According to the Big Bang theory, it took another 500,000 years for atoms to form and 300 million more years for stars and galaxies to begin to appear. Countless stars evolved and died before our own Sun and its planets came into being in our galaxy, called the Milky Way. And it was only 4.5 billion years ago that our solar system was formed from a cloud of dust and gas.

Scientists figured out that the universe began with a Big Bang. Background microwave radiation and infrared light gives clues as to how the universe was formed and how it is still expanding today. (NASA/JPL-Caltech/A. Kashlinsky)