My world has a huge void gap. How do I fix it? [duplicate]

I have this huge gap in my world. I was just starting to get things going with the FTB mod so please tell me this is easy to fix.

When I enter it I can move forward very slowly. I did not move in all the way but just a bit with SHIFT and took me some time to get out of it. I cannot build in it either. For further experimentation, I need to bring my Efficiency IV, Unbreakable III, and Fortune II diamond pickaxe to safety first, which is another reason I want this world fixed.

This is a standard problem in Minecraft, and a known bug.

Fix it by pressing F3 + A to refresh your chunks, or failing that, logging out and back in again.