The Big Bang
The Big Bang was an event that created the universe about 13.8 billion years ago. In English, we also say "big bang". The big bang caused matter and energy to suddenly expand. This created space. Astronomers cannot yet prove that the Big Bang actually happened. However, they have found evidence that supports this idea.
At the time of the Big Bang, the universe was hot and tightly compressed. After the Big Bang, the universe began to cool and expand rapidly. Within minutes, particles of matter formed atoms of hydrogen and helium. These atoms later formed stars, which led to the formation of planets. According to astronomers, the universe is still cooling and expanding.
Many astronomers believe that the universe will keep expanding. Others think that one day the universe will collapse and disappear altogether. But that will not happen for a long time yet, because the galaxies are still moving away from each other very quickly.