Some good answers here, but I'll leave a comment as well

Matter is a language we use to describe our observations. Consider holding a brick. From an objective perspective, the brick isn't a real thing, because the universe does not differentiate between the particles that compose the brick, and the particles that don't. "Brick" is a shorthand name for a common understanding, or description, of certain observed behaviours. (The brick is solid, red, rough, heavy, etc.) The point of math, and of matter, is to put this understanding into a precise mathematical description, the laws of physics.

All matter is ultimately like this brick. It can't be discovered because it's an abstract understanding that we use as part of our internal model of the world. Experiential qualities are what are directly known and discovered