Problem: Faces turn transparerent near edges

Apologies in advance if this is the wrong place for this. I tried searching for threads with other people having this problem, but had no luck - my searches weren’t helped by the fact I don’t really know how to describe the problem.

This is the first time I’ve seen this. In other words, I’ma total noob. I spent about 12 hours+ trying to make a car. Double-checking for no non-manifold meshes, and no doubles. Everything was good, but then I went for lunch, came back and noticed the problem indicated in the screenshot. Maybe I hit some button on accident, but I have no idea what. The screenshot is one of a default cube that I created (completely unaltered, other than its default position at 0,0,0). It’s in the same project as my car. Every single face of my car exhibits the same a effect as the default cube in the screenshot.

Any help would be greatly appreciated.