Second, let's understand the early predictions of such an idea.

1. One should observe that very distant astronomical objects, corresponding to faint objects (e.g. because intensity falls off with distance), are moving away from us in any direction we look.

Evidence: Edwin Hubble and others observed distant galaxies are all receding from us in every direction.

Of course, alternative explanations were offered for that observation, as in any good scientific situation. Let's look at another prediction that came after more was understood about nuclear forces.

2. Light nuclei - Hydrogen and Helium, and likely also Lithium - should all have been formed early in the universe, when it was still hot, and the ratios of those in the universe "locked in" when it cooled sufficiently.

3. Because the universe became electrically neutral and light was no longer trapped, the universe should be filled in all directions with light from the early universe.