Basically, dark matter is something that has gravitational pull, but shows no other evidence of its existence. We have seen evidence for dark matter in galaxy r

Basically, dark matter is something that has gravitational pull, but shows no other evidence of its existence. We have seen evidence for dark matter in galaxy rotation curves, galaxy cluster interactions, and the cosmic microwave background fluctuations, so we know it's not a mistake (each of these measurements are independent and do not share common systematic errors). We know it's not normal matter (e.g. black holes), because we at least somewhat understand the physics of how normal matter works, and we would expect to see something observationally if it is just normal matter. Since it is not normal matter, using Occam's razor, we can only postulate it is something that has gravitational pull but doesn't interact otherwise. Perhaps once we figure out how to study dark matter, we can tease out some of its properties and it won't just be all lumped together as "dark matter".