Momentum... What is it?

What is momentum? It travels at the speed of sound, moves through all substances, can be dissipated by transfer into an object less elastic, and can rebound back into the original object after transfer if the object absorbing the momentum cannot move. When you throw a walnut, the momentum is traveling down your arm, through your hand and then transferring to the walnut as you release it. When you stop your arm, the momentum is carried out and through the walnut, carrying it away. The only thing stopping the walnut is a collision with another object (1), air resistance (2), and gravity (3). With the first example,  much of the momentum will be lost in the crushing of the walnut's hull, dissipating into the air. some will rebound back into the walnut, causing a bounce back off the object it has met. With the second example, much of the momentum is lost directly into the air through which it is traveling because the air can absorb alot of momentum, being easily compressible and thus very much NOT elastic. The third example, gravity, does not directly affect the momentum of the walnut. The walnut, in a vacuum, will still be moving the same speed 3 minutes after the throw as it was when first thrown. But it will be moving down at such a rate as to seem, on a graph, to have slowed down to almost a freefall. Gravity does indirectly affect the walnut's momentum in that it will eventually cause the walnut to land, dissipating almost all of the momentum immediately. So what is momentum? Is it a substance that can transfer between substances? Not unless it is 4th dimensional, in which case it is carrying the object because of pressure against the force field between electrons caused by the separation of substances. Is it an energy? Many refer to the energy transfer from your hand to the walnut, meaning the momentum transfer. But if it is an energy, it would not be moving so slow as the speed of sound. Sound itself is momentum, carried through the air, transferring from one molecule to the next until it reaches our ears. Momentum is everywhere. It carries the planets in their orbits. It whirls the galaxies. It is the force that continues the expansion of the Universe itself. It is what keeps an electron in orbit. Yet we know so very little about what it actually IS.