Big Bang Theory
States that the universe began with a huge explosion 13.8 billion years ago
Real World Example
Imagine trying to set up a large party tent in your backyard; it's all packed up tightly in a small bag. As you unzip the bag and start pulling out the tent, it rapidly unfolds and expands, covering a vast area of your yard. Similarly, the Big Bang Theory suggests that the universe started from a tiny, dense point (like the packed tent) and then rapidly expanded into the vast cosmos we see today (just as the tent spreads out quickly to cover more space). Just as the tent contains all the components necessary from the start to expand into a large structure, the universe contained all the matter and energy needed to develop into its current state right from that initial explosion.
