[dantheman]

An absence of evidence for the argument against something being common is evidence.

[kelnos]

Evidence for what, though? An absence of evidence is perhaps evidence that someone who is asserting something is happening didn't do their homework. But an absence of evidence is certainly not evidence that something is not happening. It might be persuasive enough to allow people to reasonably believe that something is not happening. But it's not evidence of such.

[metabagel]

How would one go about finding evidence that retaliation is common in the U.S.?

Sometimes, absence of evidence is because the evidence isn’t readily available.