Dasam Granth is written by Tenth Master and it's authenticity cannot be doubted for many reasons the things which are explained In Sri Guru Granth Sahib are clearly explained here and it's reasons are stated in this Granth.
Why I am Saying so here is the verse from Dasam Granth Sahib where it proves it's authencity-
"I shall complete this Granth only if Thou Helpest me". ( Bachitar Natak )
Now who says that Dasam Granth Sahib is not written by Tenth Master then one should
ask these so called scholars then where this verse has come from where Sahib Guru Gobind Sahib ji Maharaj is been blessed to complete Dasam Granth and moreover the verse is clear and direct in it's message. Do so called scholars have answer to this?
It is not possible to accept something based on 1 line. Looking at the whole message is important to make an informed decision. There are too many contradictions to the Guru Granth Sahib Ji in the dasam granth to justify it