It's the question that always comes up when thinking about the origin of the Universe: what came before? And if there was no 'before', what was the cause of the Big Bang in the first place?
Until a few centuries ago, the answer was easy: some eternal deity set everything in motion. Even Isaac Newton believed that God created the Universe, some 6,000 years ago. Later, many scientists, including young Albert Einstein, assumed the Universe itself to be eternal and everlasting.
But when cosmic expansion was discovered, Belgian cosmologist (and Jesuit priest) Georges Lemaître realised there must have been a beginning - a scientific version of Genesis, so to speak.
Not that everyone immediately agreed. Well into the 1960s, Fred Hoyle's steady-state theory was quite popular among iconoclastic scientists as well as lay people. Hoyle accepted cosmic expansion, but he didn't believe in the Big Bang. Instead, he assumed that a slow, continuous creation of new matter could keep the average density and the general properties of the Universe constant over time.
The 1964 discovery of the cosmic microwave background was the major nail in the coffin of the steady-state theory. Ever since, supporting evidence for the Big Bang origin of our Universe has accumulated to a point where there's hardly any doubt left.
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