The Big Bang model is our most successful explanation for the history of the universe that we live in, and it's ridiculously easy to encapsulate its core framework in a single, T-shirtable sentence: A long time ago, our universe was a lot smaller. From this simple statement flows major testable predictions that have been verified by decades of observation. The expansion rate of the universe. The cosmic microwave background. The production of the lightest elements. The differences between near and far galaxies. All the juicy lines of evidence that makes cosmology a science.
But there are some issues. [The Universe: Big Bang to Now in 10 Easy Steps]
The "vanilla" Big Bang model, without any other additions or amendments, can't explain all the observations.
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