Lemaitre had an idea. And he, unlike most of the others, also had an explanation: The universe has a constant, homogenous mass but is constantly expanding. A little bit later, he refined this theory to say that the universe had always been expanding, ever since its earliest point, when it sprung from a single "primordial atom."
It was the most pregnant thing that ever existed.
Many scientists disagreed: Einstein said the universe was static. ("Your calculations are correct," he told Lemaitre, "but your physical insight is abominable." That's pretty much the nerd equivalent of getting served.) Others were even more dismissive and called Lemaitre, strangely, a religious nut. That's right: As counterintuitive as it seems now, the big bang theory was originally viewed as a religious description of creation.
Think about it: Creationists say that there was originally nothing, but then God created the heavens and the Earth, and light, and BBQ Pringles, and everything else worthwhile. If you squint your brain real hard, it sounds a bit like the modern big bang theory, doesn't it? Science, at the time, said that this was nonsense. The universe had always been and always would be; it had never been created. The idea that one little "atom" started it all at a defined point in history sounded a hell of a lot like a priest spouting pseudo-science to back his own beliefs.
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At least it makes Genesis a little more interesting.