One of the traits that good scientists -- or, for that matter, smart people in any field -- is that they freely admit there are things they don't understand. The risk to admitting ignorance, though, is that your opponents will take that admission and use it to suggest that you don't know what you're talking about. Richard Dawkins has a smart essay about how creationists twist such admissions by scientists.
The standard methodology of creationists is to find some phenomenon in nature which Darwinism cannot readily explain. Darwin said: “If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.” Creationists mine ignorance and uncertainty in order to abuse his challenge. “Bet you can’t tell me how the elbow joint of the lesser spotted weasel frog evolved by slow gradual degrees?” If the scientist fails to give an immediate and comprehensive answer, a default conclusion is drawn: “Right, then, the alternative theory; ‘intelligent design’ wins by default.”