Copernicus postulated that earth moves around the sun (rather than the other way around) but he was troubled about the idea. What concepts of mechanics was he missing that would have eased his doubts?

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.

3. dark matter

empty space could also be a simple vacuum