The universe is very large. There is many mysterious in our universe that we have never explored. On the other hand, the basic particle of the universe, is very small. Much smaller than we can seen by our eyes.
If your can remember, there is a movie called "Men in Black (MIB)". The movie said that is a universe exist in a small particle. And our universe is just a small particle of another world. Is the above too hard to understand. Anyway, the universe is complicated and we can never fully understand it.