Gravity pulls us to earth, a lesson you learn viscerally the first time you fall. Isaac Newton described gravity as a universal attractive force, one that holds the Moon in orbit around the Earth, the planets in orbit around the Sun, and the Sun in orbit around the center of our galaxy.
In the 1990s, astronomers made the astonishing discovery that the expansion of the universe has sped up over the past 5 billion years, which implies that gravity can push as well as pull.
