If you can answer the why, then I think physicists around the world would really like to know. We "know" the universe is expanding because we can observe the effects (eg, red shift of further away stars/galaxies), but the "why" has been the big, fundamental question for as long as it was predicted and/or observed. It's almost certainly related to some property of physics that we don't yet understand, but ultimately, we may never know any reason beyond "that's just how it is".