Origin of species
This idea was controversial when first proposed by Charles Darwin in his 1859 book On the Origin of Species but ultimately was overwhelmingly accepted by the scientific community due to ever-increasing bodies of confirming evidence across multiple disciplines and the lack of any viable alternative explanation.
Despite the now-overwhelming evidence, evolution by natural selection nonetheless continues to be opposed by religious groups who advance alternative theories proposing that most species were created in essentially their present forms by a supernatural force or entity of some kind, most commonly referred to as "God". Where these beliefs flatly contradict the available evidence, they are known as creationism.
Some versions of creationism have been carefully vetted, removing references to the Bible or God, in order to avoid running afoul of the separation of church and state mandated by the US Constitution and thence to permit their use in US public schools. Intelligent design, the best known of these attempts to insert religious mythology into educational curricula under the heading of "science", avoids (often crudely) any mention of religion or the supernatural while using scientific-sounding (but thoroughly refuted) arguments against evolution by natural selection as its primary justification.