Whether or not there is such a thing as dark matter is an empirical question, meaning it’s existence can be tested, and either proved or disproved, and is not subject to opinions, especially from someone without knowledge of physics or astronomy. We can have on opinion about the existence of God, but not about testable hypotheses such as gravity, or dark matter, or global warming. Still, I’m at least in the position to evaluate the quality of the research methodology, but most people don’t have such backgrounds. And so there is little choice but to rely on the people who research such things to give the answers.
Now that’s not to say that every scientific study is sound. Some experiments are badly constructed, the measurement tools may be flawed, the researchers may have in-going biases or are just bad at their jobs, etc., and so the results are meaningless. But that’s not for the laymen to decide.
Thankfully, scientists are pretty good at policing themselves — otherwise we’d spend a lot of time wading through crackpot ideas. One way they do it is through peer-reviewed journals. When a study is completed, the results are published in an academic journal. But before that happens, there’s a hurdle to jump. Other scientists examine the details of the experiment — e.g., the design of the research, the measurement tools that were used, the statistics that were applied, and the conclusions that were drawn. If they deem the design is valid and the analysis is appropriate and accurate, then it’s published in the journal. If it doesn’t pass muster, it’s rejected. That’s the peer review process.