Complex sentence: It is believed that the big bang occurred around 13.7 billion years ago, which is only about 0.5 billion years before when this galaxy is being seen, i.e., the galaxy is being seen at a time when the universe was only 470 million years old.
Split into simpler sentences: It is believed that the big bang occurred around 13.7 billion years ago, which is only about 0.5 billion years before when this galaxy is being seen. Also, the galaxy is being seen at a time when the universe was only 470 million years old.