DATA does not "say" dark matter exists. The indirect data we have suggests that as one possible explanation (and so far the only one that has survived critical analysis of numerous experiments).
What is dark matter to you? To scientists, it's matter we can't detect with current instrumentation. It's that simple. It does exist. Now could scientists be wrong. Yes. Absolutely. However you can't merely say there are wrong because they haven't found all the answers. If you have other plausible explanations please let them know. So far the best minds of science can't describe it in other terms other than "dark."