This is my theory at how the universe expands.
The smallest particle of matter is energy. I believe it is energy because energy can not be destroyed, it can only be changed in form when energy is grouped large enough together to create the neutrino, then grouped large enough to create quarks and so on.
So when matter is floating in space and touches anti-matter, the two do something to create energy. Which then in turn expands the universe.
Big Bang Theory: Big ol chunk of matter and anti-matter touched.