Our understanding of the birth and evolution of the universe is the result of work by hundreds of scientists worldwide.
The Big Bang theory is based on several key observations. Firstly, we know that the universe is expanding. So it’s logical that the universe was smaller in the past, and further back in time, even smaller.
Secondly, astronomers can also observe a kind of afterglow of radiation left over from the Big Bang, known as the cosmic microwave background. This radiation contains important information about the conditions of the early universe.
What’s more, hydrogen and helium can be found in the same ratio across the universe, and the only explanation is that they were created at the same time, at the beginning of the universe.
Finally, we know that the universe, and the age of some of the oldest objects in the universe, are about the same.