In physics mass and energy are interchangeable. A little bit of mass can be converted in principle to a large amount of energy or a large amount of energy can be converted into a tiny bit of mass.
For example, a massive star can collapse into an event horizon of a black hole. A large amount of energy can also theoretically be concentrated into a small enough space to create an event horizon of a black hole (known as a kugelblitz).
Depends what you mean by "immaterial"; a photon of light is considered immaterial because it has no rest-mass. A simple solar cell turns the photon energy into electrical energy. Everything around us is an interplay of energies, some material, some immaterial.
Are thoughts material or immaterial? You can create many things with your thoughts.
When a large nucleus splits into two smaller ones, the sums of the new masses are greater than the original nucleus. Where did that mass come from? From the immaterial energies that hold nuclei together "releasing" some of that energy as mass.