The Argument From Ignorance, also known by its Latin name argumentum ad ignorantiam, is the fallacy of assuming something is false if not proven to be true.
The Argument From Ignorance is commonly employed by those opposed to a particular scientific theory. For example, creationists frequently argue that because scientists cannot explain the evolution of a particular feature (at the time of writing, the hummingbird's tongue is the usual choice), the Theory Of Evolution must be false.
To show the fallacy has occured, argue that the proposition may be true even though its truth has not been demonstrated. In the above example, we could point out that creationists once used (and some still use) the example of the eye, and it wasn't until 1994 that Daniel Nilsson and Susanne Pelger used computer simulations to show how the eye can evolve from a single light-sensitive cell with improvements at every stage.
The Argument From Ignorance is a special case of The False Dilemma - it assumes something must either be known to be true, or known to be false.
A note on 'innocent until proven guilty'
The principle of 'innocent until proven guilty' is not an example of The Argument From Ignorance. Despite its appearance, it is not a statement about truth, but about how we should act - that we should treat someone as innocent until they are proven to be guilty. Obviously a trial does not change someone's property of being innocent or guilty.