Hard to say but there are many ways scientists thought it happened. . The general idea is that there was a large concentration of matter in an extremely small space. Eventually, the matter began to expand at a large rate, which has been observed (the expansion of the universe has been observed.). But what exactly caused the Big Bang is up for debate. All we know is that the universe was at one point small then started to grow exponentially. As we learn more about space and physics we'll uncover what caused the Big Bang. This, of course, is if we're able to comprehend it, like how we can't comprehend non-existence.