Q:

What's "exchange dark looks with somebody" mean

I've read a sentence "He exchanged dark looks with his wife". But I cannot understand what does the expression "exchange dark looks" mean even though  looking up for dictionaries. 
I'll appreicate that if you could help me. 

A:

A context would help. It's clear from the example that you've given that it's from a piece of literature. The only example I could find was from Chapter 1 of Harry Potter and the Chamber of Secrets. Here is the full excerpt that gives light to the meaning:

"Do I look stupid?" snarled Uncle Vernon, a bit of fried egg dangling
  from his bushy mustache. "I know what'll happen if that owl's let
  out."
He exchanged dark looks with his wife, Petunia.

Harry wants his owl let outside, because he says that it's bored. Uncle Vernon, however, knows exactly what will happen if this is allowed and he is expressing fear when he exchanges 'dark looks' with Petunia. They are Muggles, after all.

