The author of
The Elegant Universe, Brian Greene,
pays tribute1 to the hundred year anniversary of Einstein's famous equation:
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.
The guiding hand of Einstein's E = mc² will have finally come to rest.
Now that's a happy thought. Fortunately my molecules will have long redistributed before that.
[1] Via the New York Times. Try BugMeNot if it's asking you to log in.