A great deal of fascinating and illuminating science has gone into understanding the Big Bang and the formation of the universe as we know it afterwards. One of the most famous of these was the investigation of the cosmic microwave background radiation, energy left-over from the Big Bang which is present throughout the universe. I suggest you look it up for some fascinating insight into how scientists can look at the universe as it is today to figure out how it was billions of years ago. This might be a good place to look, maybe with help from a teacher:

astro

But trying to understand what happened before the Big Bang is problematic. Science is the skill of looking at patterns in the world around us to find natural laws, then figuring out how those natural laws (like gravity or the speed of light) will interact in different situations. For example, you might notice that whatever you throw in the air falls to the ground. From that, you eventually understand how gravity works, and can predict that anything anyone else throws in the air will fall, too.

But at the Big Bang, the natural laws, like the speed of light, weren't set in place yet. They were changing, and that makes it almost impossible to figure out what was happening at the moment of the Big Bang. And also because of that, our current science gives us no way to probe cosmological questions about what might have been before the Big Bang.

Many physicists and mathematicians spend a great deal of time trying to find ways around our limitations in what we can deduce, and to of physics that says that there are other dimensions which we can't see, but which are all around us, has sometimes proposed that colliding universes might be the cause of the Big Bang. But there's no way for us to test and confirm those ideas yet. So, really, no one knows for sure what happened before and caused the Big Bang.