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.
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