Scientists are pretty sure that dark matter is not just regular gas and dust because the amount required to create the gravity we see, would be visible. It would block or reflect a lot of the nearby starlight.<p>Just on the back of an envelope: If we assume the percentages in my post above apply to an individual galaxy, then there has to be 5x as much dark matter mass as lit mass. There&#x27;s no way you could have 5x as much gas and dust in a galaxy as stars, and not see it.<p>For comparison, the sun makes up about 99.8% of the Solar System mass (500x as much mass as all the planets, dust, etc. combined).