More than 60 million people were killed during the Second World War, of which nearly seven million were Jewish.
Yet online there are commenters who deny it ever happened at all.
William Allington, a PhD Candidate in Jewish Civilisation at the University of Sydney, has focused his study on the nature of Holocaust denial, especially its modern online forms.
"Holocaust denial is the denial of the plan to exterminate the Jews, the denial of the machinery, the gas chambers, and the demographic data proving the exetermination of Jews in the final solution," Mr Allington said.
But the denial of the Holocaust is used to serve a greater purpose, Mr Allington said.
"It's self-serving to whomever is dening it - it relegitimises national socialism, it's used to discredit the state of Israel, and rekindle Anti-Semitism," he said.
"In fact, Holocaust denial is a modern form of Anti-Semitism."
Mr Allington said Holocaust denial was a worldwide phenomenon.
"A lot [of it comes from] the Middle East, yes, but most of it comes from the West - UK, Australia, America, Germany," he said.
This worldwide spread of Holocaust denial, especially in the 21st century, has been facilitated by the advent of social media, Mr Allington, whose study focuses mostly on Reddit, said.
"In the last five years, Holocaust denial has moved online, especially on Reddit," he said.
"With the rise of social media [Holocaust deniers] carve out little area where they broach discussions."
The online Holocaust movement sees its foundations in written and published literature, Mr Allington said, which is then used as reference material for online debates.
"[Holocaust deniers] published and wrote pseudo-intellectual books, articles, and journals," he said.
"As a result, to the trained eye it was nonsense, but it created the reference material to get any untrained eye to believe it."
As an example, Mr Allington points to American historical figure Willis Carto, the founder of the Liberty Lobby (a Holocaust denial group) which was affiliated with the Ku Klux Klan and used Holocaust denial as an extending form of propoganda against Jewish people.
"That's continued as a strain within online American communities," Mr Allington said.
Online Holocaust deniers gain traction through subreddits or by posting tangential comments on vaguely related, and "fairly benign" topics, Mr Allington said.
"Usually with posts on historical areas someone will post something tangential, like 'Did Hitler actively kill the Jews', to instigate a reaction," he said.
Mr Allington likened these commenters to "trolls", commenters who aim to ignite controversy online.
However these Holocaust deniers have greater ambitions than simply stirring the pot - they want to recharge Anti-Semitism and destabilise the Jewish community, according to Mr Allington.
But given the amount of studies and details we have concerning the Holocaust and its existence, it seems at odds for such a large community of online users to be denying it, he said.