Yes, yes, I know the title looks impressive!

Ok, so you’ve been reading about how the redshift explained the beginning of the universe. If we go back up in time, galaxies start to stick back up together (instead of spreading, they regroup, until they’re just a dense hot mixture of particles), and 13,7 billion years ago, the universe was only a big soup of elementary particles interacting with each other. As the expansion of the universe started, the universe (which was so dense that it was several million degrees hot), cooled down, and the more it expanded, the more it cooled. This cooling allowed atoms to start to form out of the elementary particles that were present in the big soup.

However, not any random atoms were made, but only the simplest of them. Remember that in physics a system always tends to its simplest way of existing, and the two simplest elements are hydrogen and helium. If you look at the composition of the universe today, you’ll notice that over 70% of the matter building it up are either hydrogen or helium. Does that ring any bells? 1 more point for the Big Bang Theory!