Dark matter/energy are the names given to energy / material that we cannot explain but must be there to explain what we do observe. Only 5% of the universe matter is in the form of atoms, the rest is Dark. For example if you do the Maths on a Galaxy you will see that there is not enough mass for cohesion, yet they stay together, the absent matter needed for gravity to hold he lot together is Dark. Geneall it is though that dark matter holds things together on a galactic scale, dark energy pushes the galaxies apart.