Matter is potential energy. It can be converted to other forms of energy, like kinetic energy, heat, or light. It is essentially the energy required to keep the mass of the object stable.
What makes this energy? Further down, what makes up all substance? That's the illusive nut String Theorists have been trying to crack. In a very off-the-cuff way I would describe it as vibrations having harmonic resonances which push energies into concentrations to form matter. But the quantum world is a lot more complex than this explanation.
What is this "energy"? Even further down? On a philosophical or spiritual level? Maybe it's the difference, or the interaction of and friction between, something and nothing. Left and right, up and down, on and off, 1's and 0's, Yin and Yang, there are a lot of analogies which come to mind of simple 2-choice alternatives. Maybe it is simply the interaction which causes vibrations and begins the process of creating substance.
....yeah, maybe not... but it sounds pretty.