Black weird faces

Hi. I have a problem. I modelled a mesh, added the mirror modifier, accepted, joined the two halfs then merged the remaining vertices manually. I got weird black faces. How to remove them? They can’t be selected.

. Tried to recalculate normals, as well as deleting doubles and…nothing.

Divide up the large faces to get rid of the ngon.

You’ll need to share your blend.

Thanks. Adding some edges solved the problem. However, I am wondering why this is happening?

Simplified: you need 3 points in space to clearly define surface. There would be only one fitting. As soon as there are more points there is uncertainty for the PC and they do not like this. Quad is easy - 2 triangles, so you would get this faint line where diagonal would be if points are in different planes; ngons are much more complicated to do such guesswork and thus you get these.
Define your surfaces, make pc not suffer.