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. It is so much easier to assume that it was there forever. But the data were not to be denied.