Things fall towards the earth. You can check that they do right now by dropping something. That things fall towards the earth isn't a theory - it's an observation. Gravity - the most popular explanation for why that happens - is a theory!
"Dark matter" comes from a group of observations that consistently suggest there is more mass in the universe than we can account for. The fact that we keep observing it is not the same thing as an explanation for the observations. There are also theories that try to explain dark matter! But the fact that people observe it is not a theory.
"Dark matter" comes from a group of observations that consistently suggest there is more mass in the universe than we can account for. The fact that we keep observing it is not the same thing as an explanation for the observations. There are also theories that try to explain dark matter! But the fact that people observe it is not a theory.