What does the phrase "You don't say" represent?

Solution 1:

"You don't say?"

informal

1. used to express amazement or disbelief.

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Bob could have easily have simply said, "really?"

Solution 2:

You don't say is an idiom and can mean several things depending on context. Could range from sarcasm or irony to mild surprise or amazement. The hearers usually have to supply the interpretation for themselves.

How surprising, is that true? Also, I find that hard to believe. For example, I've been working on this project for two years.-You don't say, or The man who runs this soup kitchen is a real saint.-You don't say! This expression, a shortening of you don't say so, may be used straightforwardly or ironically. [Late 1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.