"Where did we come from?" - is the oldest and the most asked question by philosophers, mathematicians, physicist and even poets. There's no confirmed reason in science but majority scientists believed that we began from at a moment known as the Big Bang, 14 billion years ago. But how do we find any clue that this actually happened? I will show some.
The key of the theory of Big Bang is expanding. Every material was once gathered together. After the occurrence of Big Bang it expanded and formed many stars and galaxies. When we look at the night sky, the stars we can see are within our own galaxies. There are some fuzzy patches too, which other galaxies. Almost all of this are moving away from us at incredible speed - some at hundreds of thousands of kilometers every second. This means the Universe is expanding.
After the Big Bang, the whole Universe was flooded with incredible bright light. As the Universe expanded, that light stretched into microwaves. That's why an afterglow of the Big Bang can be captured, by microwave telescope. Infact a view through a microwave telescope shows the whole sky filled with a glow. This glow is called Cosmic Microwave Background. Unlike the light coming from stars, cosmic microwave background have same colour and intensity wherever you are in the space.
Looking in space is like looking back in time. The light from distance object takes a long time to reach. SO it means if an object is a million light years away, it looks as it was million years ago. If Big Bang actually occurred, we would see gas clouds made of hydrogen & helium at a distance in space. These gas clouds are supposed to convert into stars. Recently, astronomers have found gas clouds like this in the distant Universe. Some of them are 12 or 13 years old. Summing up all the reasons, we can come to a resolution that Big Bang actually happened.