Dark matter is theoretical. We use it to explain the mass deficiency problem, but we know fuck all about it. Basically, the mass constituting all observable matter in galaxies, like our own, doesn't exert the gravitational force to hold it all together; galaxies should fall/drift apart according to the laws of gravity, so some scientists propose there's some form of matter that we can't observe, that matter being dark matter. There's additional evidence for dark matter in the form of gravitational lensing.
Antimatter is something else. For every quark and lepton on the standard model, there is an antimatter equivalent. The positron, for example, is the antimatter equivalent of the electron.