no one knows for sure, but at the top of this page is see an advertisement thing that says that dark energy and dark matter explained by quantum physics theory.
i have an idea of my own that says dark energy and matter are really just what matter and energy are actually the decay of regular matter and enery as we know them as. and that since matter is constantly growing older, and matter is constantly becoming dark matter, the universe is constantly expanding.
dark energy and dark matter are what we think, to be what is causing the universe to expand, and what will eventually cause the universal implosion known as the big crunch, so its probably pretty massive.