The Big Bang Theory is an idea about how the universe started. Scientists support this theory with a few important pieces of evidence:

1. Cosmic Microwave Background Radiation (CMB):

   • This is radiation that fills our universe.
   • It was found in 1965 and has a temperature of about 2.7 Kelvin.
   • Scientists believe it is leftover energy from the early days of the universe, matching what the Big Bang model expected.

2. Redshift of Galaxies:

   • When we look at distant galaxies, we see that they are moving away from us.
   • On average, these galaxies have a redshift (which is a way to measure how fast they’re moving) of about 0.7 for galaxies that are around 3 billion light-years away.
   • This shows us that our universe is expanding, which supports the idea that everything had a single starting point.

3. Abundance of Light Elements:

   • The theory predicts that in the first few minutes after the Big Bang, the universe created different amounts of light elements like hydrogen, helium, and lithium.
   • Today, we observe that about 75% of the universe is hydrogen and about 25% is helium, which matches these predictions closely.

Together, these pieces of evidence strongly support the Big Bang Theory as the best explanation for how our universe began.