The universe is very huge, and it's been around for a very long time. Thinking about how it all started is hard to imagine. A theory suggests that how it all started which is known as Big Bang Theory to is. But, what exactly Big Bang Theory is?
This all started in 1927, when an astronomer named 'Georges Lemaître' had a big and interesting idea. He said that a very long time ago, the universe started as a single point (this point probably be a singularity, that an another theory). Although, if you're curious to know about singularity click here. He said the universe stretched and expanded to get as big as it it now, and that it could keep on stretching.
Just two years later, after this, an astronomer named 'Edwin Hubble' noticed that other galaxies are moving further away from us and that's not all, the farthest galaxies were moving faster than the ones close to us. This meant that the universe was still expanding, just like the Lemaitre said.
If things were moving apart from each other in universe, it meant that long ago, everything had been close together. Every single thing in the universe today weren't there at the beginning. Where did they come from?
When the universe began, it was just hot, tiny particles mixed with light and energy. It was nothing like what we see now. As everything expanded and took more space, it cooled down. The tiny particles grouped together. They formed atoms. Then those atoms grouped together. Over lots of time, atoms came together to form stars and galaxies.
Bigger atoms and groups of atoms were created by the first stars. That led to more stars being born. At the same time, galaxies were crashing and grouping together. As new stars were being born and dying, then things like asteroids, comets, planets, and black holes formed.
How long did all of this take? Well, we know the universe is 13.8 billion years old –that's 13,800,000,000 which is a long period of time. That’s pretty much how the universe began. Because it got so big and led to such great things, people call it the "Big Bang."
In short, The big bang is how astronomers explain the way the universe began. It is the idea that the universe began as just a single point, then expanded and stretched to grow as large as it is right now—and it is still stretching!
