Dark Mystery No. 2: What Is Dark Energy (and Why Is It So Weak)? (String Theory)

There’s a lot of dark energy in the universe — about three times as much as visible matter and dark matter put together! This energy represents a repulsive force of gravity on large scales, pushing the edges of the universe apart.
The abundance of dark energy by itself isn’t so much a problem; the real problem is that the dark energy is a lot weaker than physicists would expect from purely theoretical calculations based on quantum field theory. According to those calculations, the random energy of empty space (the “vacuum energy”) should explode up to huge quantities, but instead it maintains an incredibly small value.
A theory of everything would hopefully explain why the vacuum energy contains the value it does.