Is there any evidence that the holocaust did, or didn't, happen?

In a word? No. Most Holocaust denial is always predicated on conspiracy theories of some sort or at best picking at small parts of the larger narrative around the Holocaust in order to plant seeds of doubt.

If you're looking for resources this site is devoted to debunking all the common points brought up by Holocaust deniers. There is also the book Denying History: Who says the Holocaust Never Happened and Why by Michael Shermer

Also just to add, no actual historians deny the Holocaust in the modern era. There was a period directly after the war where some historians did put forth theories that tried to deny parts of the Holocaust, but in the modern era most Holocaust denial works are done by non-historians like David Irving (journalist), Fred Leuchter ("Scientist"), etc.

Edit:

[The Nizkor Project] (http://www.nizkor.org/) is also a very good online resource.