Sometimes, I might say 'Man', as the precursor to a statement as in this recent example I said to myself after reading something: "Man, to give anything a label will always technically be reductive, and thereby cause assumptions..." or "Man, there are a lot of people here."
I don't like using 'man' since it sounds too casual to me, and I've never used 'gee' since it sounds outdated. I sometimes use a 'Hmm,', but it sounds pretentious. "Eureka" eureka wouldn't quite fit the situation here.
To simply omit the exclamation changes the implication of the sentence, indicating it is a fact, rather than something that came from your thoughts.
Right now, my choice of interjection would be a brief "Hnn,". Can anyone suggest a nice more 'scientific' alternative, maybe something you've seen on a show, read an intellectual say, or use yourself?
Side Note: Google's dictionary (oxford pocket) doesn't actually list 'gee' as a synonym for 'man', maybe this should be added.