The discovery of red-shift in light from distant galaxies led to the development of the Big Bang theory. This was a period of cosmic inflation that lasted mere fractions of a second — about 10 Theories about the development of the Universe, such as the Big Bang theory, are based on astronomical observations and ideas such as red-shift and dark energy. While the majority of the astronomical community accepts the theory, there are some theorists who have alternative explanations besides the Big Bang— such as eternal inflation or an oscillating universe. Astronomers can, however, see the "echo" of the expansion through a phenomenon known as the cosmic microwave background.

The Big Bang Theory: The Complete Series [DVD] [2007] [2020]

The discovery of the CMBR, after it had been predicted by the theory, provided very strong support for the Big Bang theory.According to the Big Bang close Big Bang The scientific theory that describes the early development of the Universe. Simply put, it says the universe as we know it started with an infinitely hot and dense single point that inflated and stretched — first at unimaginable speeds, and then at a more measurable rate — over the next 13. Suddenly, an explosive expansion began, ballooning our universe outwards faster than the speed of light. The CMBR is the remains of the thermal energy from the Big Bang, spread thinly across the whole Universe.

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Existing technology doesn't yet allow astronomers to literally peer back at the universe's birth, much of what we understand about the Big Bang comes from mathematical formulas and models. Astronomers have discovered that, in general, the further away a galaxy is, the more red-shifted its light is.