So mouse balls, the kind inside computer mice, not real mice (!!!), are very heavy, but what inside them makes them heavy? To answer that question, I cut one of them open with a box cutter. Then, I scientifically observed the contents, and came to the conclusion that mouse balls have a steel ball bearing inside them to give them their weight. I observed that the metal sphere inside the rubber outer coating was 28 grams in weight and was dull silvery in color.
The sphere was magnetic as well. I then calculated the diameter with a pair of calipers, then halved this to get the radius. By cubing this, then multiplying it by 4/3pi, I got its volume in cubic centimeters. Then, by dividing its weight by its volume, I got approximately 7.03 grams/cm^3 as its density. This seems a little bit light for steel, but the material was magnetic. Select other elements are magnetic, but most elements with similar densities (~7 g/cm^3) don't make sense for a mouse ball. Who makes trackballs out of neodymium? Therefore, without other data, it is reasonable to conlude that it is indeed steel inside a computer mouse ball.