Also armchair, but this is how I understand it: if you take gravitation as it is typically understood, it implies motion of stars around galaxies which we don't see. The discrepancy can be accounted for if you assume there is 33% more mass than we can traditionally observe, roughly suffusing each galaxy. Since we can't observe it, it's believed to not interact with any normal matter other than through the gravitational field.

edit: 33% is definitely wrong, but I don't know the actual number...