1. If "not matter" means not composed of ordinary matter, then dark matter does not matter. Dark matter is a type of matter that is not made up of the same particles that make up ordinary matter, such as protons and neutrons. Rather, it is thought to be made up of exotic particles that have not yet been detected.
2. If "not matter" means not important or significant, then that can be debated. Dark matter may be composed of exotic particles that do not interact with ordinary matter or light, making it difficult to detect. Nevertheless, it is believed to make up about 27% of the universe, and it plays a fundamental role in the dynamics of galaxies and clusters of galaxies. In that sense, dark matter does matter.
