According to quantum field theory, vacuum is not nothing, but probably contains an enormous amount of energy. A primary effect of this energy should be on gravitation on a cosmological scale. In fact, in 1917 Einstein introduced something very similar into his gravitational field equations, the so-called “cosmological constant”, to overcome what he thought was a flaw in the equations. He soon came to regret the change he had made, calling it “my biggest blunder.” Was it a blunder or not? Recent observations show that the cosmological constant seems to be there after all, but its observed size is so far removed from the predictions of quantum field theory that its nature poses the most far-reaching unsolved problem in physics. Does its observed size have anything to do with the fact that we are here to observe it?