As much as 90% of the matter in the universe my be unseen i.e. dark matter. There is a variety of explanations offered by many astronomers and physicists for this dark matter. It could merely be ordinary material such as ultra-faint stars, cold gas, large or small black holes or dust scattered around the universe, all of which emit or reflect too little radiation for our instruments to detect. It could also consist of exotic, unfamiliar particles that we have not figured out how to observe.