Astronomers believe that at around 15 billion years ago, the universe was born in a big explosion, and that started the expansion of the universe. In 1927, a German priest called Georges Lemaitre was the first to suggest what is now known as the â€˜Big Bang theoryâ€™.
The early universe began very small, dense and hot, and after it was born in the big bang, inflation (a massive expansion for the universe) happened in just one-hundred-million-billion-billion-billionth of a second. During the Big Bang, all forms of energy and matter were formed, including space and time itself. After this ended, the universe continued to expand but much more slowly. 1 second later, it began to cool and some basic forces emerged (gravity, electromagnetic forces, etc.) 3 minutes later, protons and neutrons came together to form simple elements (hydrogen and helium). And for the next 300,000- 500,000 years the universe remained a hot enormous cloud of gas that was still slowly expanding. And 1 billion years later, stars and galaxies began to form, starting to create what is now our universe.
In 1929, Edwin Hubble discovered that the universe is expanding, and that galaxies are moving away from us. This supports the Big Bang theory. Also, in 1965, radio and microwave astronomy experts Robert Wilson and Arno Enzias picked up a faint microwave signal when they pointed a giant antenna at the sky. Itâ€™s believed that the signal is the remainder from the heat of the Big Bang. This also supports the Big Bang theory.
Overall, the Big Bang theory is the best theory for the birth of our universe.