The Big Bang theory, strongly supported by the cosmic microwave background and the cosmic expansion rate, says that starting 13.8 billion years ago, everything initially raced away from everything else like an inflating balloon. The expansion was explosively rapid at first. But the gravitational attraction of every galaxy on every other kept tugging at this expansion, slowing it down. The big question of the 20th century’s last half was: Will everything come to a stop in the far future