The holocaust was a secret, it was a huge conspiracy among the German elite to carry out mass-executions of jews and others in the camps, a theory about which managed to circulate among the German population and elsewhere, but was doubted and contested. Until suddenly, allied armies liberated the camps and revealed to the world what was happening in them.

This is not to say that pogroms that happened earlier on were conspiracy theories, people knew about the pogroms that occurred as the nazis came to power, and afterwards. But the mass-extermination in camps was not widely known.

So it existed first as conspiracy and conspiracy-theory, before suddenly becoming a matter of public record.