Dark matter is a hypothesis, and one that makes pretty strong predictions, several of which have come true, so it's not inappropriate to call it theory.

The phenomenon of galaxies demonstrating significanty more mass than their sum of normal matter is the observation that is explained by dark matter.

Sorry but that’s an intellectual dodge. A verbal fiddle. Saying it is a “name for something” doesn’t make the fact that it’s a hypothesis disappear. One is simply positing the need for more “Russian dolls” inside the Russian doll one has hypothesised to begin with.