Final answer:
The person who first proposed the Big Bang theory was Georges Lemaître, a Belgian astronomer and Catholic priest.
Step-by-step explanation:
The person who first proposed that the observed motion of galaxies away from Earth originated with a colossal high-temperature explosion is Georges Lemaître. He was a Belgian astronomer and Catholic priest who developed the concept of the Big Bang theory in the early 20th century. Lemaître's theory suggested that the universe began from an extremely hot and dense state and has been expanding ever since.
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