There must be matter we cannot see, some dark matter. If true, then some 84% of the matter in the universe is dark matter. But how confident are we that Dark Matter is the answer to the anomaly of the galactic rotation curves? If, for example, we discovered the dark matter particle, with all the right properties, that would clinch the dark matter hypothesis. But yes, there is a question mark next to dark matter until we actually discover the dark matter particle.