The galaxies are racing away from each other. And if the galaxies are moving apart, then they must have been closer together in the past. Theoretically, we can run the movie backward using the laws of physics to tell us what happens. The galaxies converge. The density of matter increases and the temperature soars.
Atoms dissolve into their constituent parts.
Mass becomes pure energy. Time goes to zero. The universe begins. In general, astronomers were not happy with what the data were telling them. An eternal universe may be hard to imagine, but a universe that has a beginning is even harder to imagine.