"There seems to be a mysterious force in the universe, one that overwhelms gravity on the grandest scale to push the cosmos apart. . . . We call it "dark energy," but that name, like "dark matter," is merely a code word for our ignorance. It's okay not to know all the answers. It's better to admit our ignorance than to believe answers that might be wrong. Pretending to know everything closes the door to finding out what's really there."