Big Bang

Scientists believe that everything in the Universe began about 13 billion years ago in a giant explosion called the Big Bang. Over billions of years since the explosion, the Universe has expanded, and cooled, while matter formed into galaxies, stars, and planets. The observable Universe now measures 75 quintillion miles in all directions and continues to expand.

When: about 13 billion years ago
What: explosion that began the Universe
Temperature: trillions of degrees
Source: a tiny point of energy, smaller than the period here.
Result: observable Universe formed
Significance: only explanation of the beginning of time supported by scientific evidence

Image credits: main image: courtesy of AMNH; Charles Liu: courtesy of AMNH.