Mobile view has a gap on the right

I know a little bit about CSS and I’ve been working on a website with a friend of mine.

http://leidenmagazine.com.au/

We’ve notice that in mobile view there is a space on the right of the website, indicating an element is sitting out of the width of the normal 100% for the smaller mobile screens. It occurs on all pages, not just the home screen. I have inspected what feels like every element on the pages and I can’t see anything that sits over the screen width to cause the gap on the left.

I’m hoping someone who knows a bit more about CSS could help me out with this problem?

Thanks

https://wordpress.org/plugins/css/