The concept of a Prime Mover was originally formulated by Aristotle. Movement in his definition included a great many processes – physical movement from place to place, but also change, growth, melting, cooling, heating…etc. He argued from the principle of cause and effect that behind every movement there must be a chain of events that brought about the movement that we see taking place, and since we cannot continue backwards indefinitely, the chain of events must lead back to something which moves but is itself unmoved: The Prime Mover.
The Prime Mover is a being with everlasting life, and immaterial. Aristotle calls this being 'God'.
The Prime Mover, having the ability to move but be unmoved, exists outside the Laws of physics, one of which, Newton's Third Law, states 'To every action there is an equal and opposite reaction'. The Prime Mover may move, but nothing may move The Prime Mover.
The Prime Mover concept allowed for a while, the coexistence of science and religion without contradiction. When Newtonian mechanics was formulated, it seemed the Universe was unchanging, so that the stars would shine forever, and the planets orbit the Sun forever. When people asked, 'How did this pass?' it seemed consistent that the motion could be started with a single push and would then continue, according to the laws of Newtonian mechanics. Even today, some in the Church argue in the same sense, that though the Big Bang my have given rise to the Universe, the Big Bang itself required a cause which they attributed to God, the Prime Mover.