Oh wow, this is a really tough question for me. I’m a nanomaterials physicist, and cosmology deals with things far far far smaller and far far far BIGGER than I’m used to dealing with 🙂
It’s probably one of the hardest questions anybody can ask. For almost one hundred years, we have known that the universe has been expanding. We can trace this expansion backwards, through to the very beginning when the universe occupied an infinitesimally small point in space. This was the state of the universe over 13 billion years ago.
It is from this starting point that all matter came into existence: stars, galaxies – even space and time itself. At time t=0, this point began an unprecedented inflation, in this instant time and space were born. This event has become known as the Big Bang.
What caused the Big Bang? There are lots of possibilities! The answer might even be one we aren’t able to understand. Who’s to say it was caused by anything at all? Our brains have evolved to assume that everything has a cause, we can’t imagine any event ever not having one. The answer to the cause of the universe will almost certainly be something very very strange.