Concept:
The Big Bang Theory explains the origin and evolution of the universe.
It proposes that the universe began from an extremely hot and dense state about \(13.8\) billion years ago and has been expanding ever since.
Step 1: Understand the Big Bang Theory.
According to this theory,
• Space itself is expanding.
• Galaxies move away from one another.
• The expansion is supported by Hubble's observations.
Step 2: Identify the correct phenomenon.
Therefore,
\[
\boxed{\text{The Big Bang Theory is related to the expansion of the universe}.}
\]
Hence, the correct option is
\[
\boxed{(A).}
\]