Dark matter does not interact electromagnetically. That means we cannot see it. But its there, contributing to the mass of the Galaxy and to the speed at which stars orbit the center of the Galaxy. Look closely at the Galaxy below. Each dot is a star the color of each dot reflect a realistic temperature and mass distribution for stars in a Galaxy like ours.

Click on the kind Galaxy you want to get the stars moving: with or without dark matter. Look closely at the rotational velocity of stars on the outskirts of the Galaxy below: they go faster in a Galaxy with dark matter.