ELI5: big bang

The Big Bang is the moment long ago when our entire universe was born, starting out tiny and hot, and then stretching out very fast to become the huge space we see today.

Imagine you have an unblown party balloon, but it is squished down smaller than the tip of a pencil. Inside this microscopic balloon is everything that will ever exist: all the light, all the heat, and all the stuff that will one day make planets, trees, and people.

Then, about 14 billion years ago, someone blew into that balloon. It did not pop or blow up like a bomb. Instead, it suddenly expanded, or stretched out, super-duper fast.

Here is what happened next, step by step:

• Cooling down: In the very beginning, the universe was blazing hot, like the hottest soup you can imagine. As it stretched bigger and bigger, that hot soup began to cool down.
• Making building blocks: As the universe cooled, tiny pieces of matter came together like microscopic building blocks.
• Lighting up: These blocks clumped together to make giant burning balls of light, which are the very first stars.
• Building neighborhoods: Billions of stars swirled together to make giant star-neighborhoods called galaxies.
• Making home: Around one ordinary star, rocks clumped together to make planets, including our home, Earth.