The strongest piece of evidence that supports the Big Bang Theory is the

• A. Detection of the cosmic microwave background radiation.
• B. Observations of distant galaxies.
• C. Discovery of dark matter.
• D. Detection of gravitational waves from early universe.

Answer: (A)

The strongest evidence for the Big Bang is the relic radiation left over from the early hot universe. This cosmic microwave background is a nearly uniform glow that fills the entire sky with a precise blackbody spectrum at about 2.7 Kelvin, plus tiny temperature fluctuations. Those features match exactly what the hot, dense early universe would produce and provide a direct snapshot of the cosmos when it first became transparent, long before stars and galaxies formed. That single all-sky signal ties together the expansion, the cooling history, and the initial conditions in a way that other observations don’t match as tightly.

Observing distant galaxies shows the universe is expanding, which supports the overall Big Bang picture, but it isn’t the direct fingerprint of the early universe that the CMB provides. Discovering dark matter informs us about what the universe is made of, not about its origin. Detecting gravitational waves from the early universe would be highly informative, but such detections are not yet established with the same level of certainty as the CMB.