In the far, far future, essentially all matter will have returned to energy. But because of the enormous expansion of space, this energy will be spread so thinly that it will hardly ever convert back to even the lightest particles of matter. Instead, a faint mist of light will fall for eternity through an ever colder and quieter cosmos.Now that's a happy thought. Fortunately my molecules will have long redistributed before that.
The guiding hand of Einstein's E = mc² will have finally come to rest.
 Via the New York Times. Try BugMeNot if it's asking you to log in.