I’ve tried to extend the antimatter theme and create a new template. I created a template file and the site content with .md. However I could not manage to get any content although I simply copied the standard template. Did I miss something? Do I need to create an additional css file?
All you should need to do is copy the template file from antimatter/templates/default.html.twig into yourtheme/templates/yourtemplate.html.twig and then ensure you use yourtemplate.md as the filename for your page content.
If that is not working for you, can you zip up what you have in your theme, and share it on dropbox or something so I can take a look?
Ok, I obviously made a mistake last time, although I have no idea what it was. After copying the template again as you said it worked. Thanks for your quick reply 
Glad you got it working!