Here is what a bunch of dictionaries say.

"a theory that explains an event or set of circumstances as the result of a secret plot by usually powerful conspirators."

"a belief that some secret but influential organization is responsible for an event or phenomenon."

"a belief that an event or situation is the result of a secret plan made by powerful people."

There was never a conspiracy theory about the holocaust. Hitler spoke about the perfect Arran race in his book "Mein Kamph".It was well known. That was in 1925. He laid it out. He explained why he hated the Jews and more. It was no secret.

You can also read my late father's book.

https://www.amazon.co.uk/Memoirs-Jewish-Journalist-Nazi-Germany-ebook/dp/B0057ZEP18

People regard conspiracy theories as nutcases because they come up with things like 'the illuminati' being responsible when they don't exist anymore, that NASA didn't land on the moon, that the earn is flat, etc.

I'm sorry you have a problem with that.