Some theories of the origin of the universe predict an entire class of dark matter. In the same way an open-minded materialism would not deny the possibility of telepathy, psychokinesis, clairvoyance, psychic healing and so on. It would simply assert that, if these things are one day scientifically established, then they must have some kind of material basis. In other words, if they exist, they are forces or forms of matter which we do not yet understand. Whatever exists is matter.
Nothing can exist unless it is material.
Nothing can be perceived, nothing can exert any force or bring about any change in the real world, unless it has a material basis. Matter is not base. Many people still have a feeling that matter is somehow inferior or base, and that only mind is noble and refined. Many feel that if the mind, or life, or creation, are explained in terms of material processes, they are somehow devalued or debased, in the same way that Creationists feel insulted by the idea that we have evolved from apes. This feeling is a remnant of Platonic, Gnostic, and Manichean traditions, all of which regarded matter and the life of the body as evil.