Richard Simmons GaGa Over Anderson Cooper!

6/19/2009 12:44 PM PDT BY TMZ STAFF
Anderson Cooper was outed as a Lady GaGa fan last night -- and the eternally short-shorted Richard Simmons didn't miss a beat challenging him to a "Just Dance" dance-off.

Cooper didn't take the bait, but the 100% authentic awkwardness is just as entertaining ... if not more.