Short answer: the concept of matter is too vague. Before I could say whether I would be a materialist, I need a clear and precise definition of what is understood by “matter”. As far as I know, there is no single, widely accepted definition of matter.
Long answer: there are two issues which cause me to refrain from fully accepting materialism. These issues are (1) dark matter and (2) consciousness. I will give a short discussion of these issues.
Astrophysicists believe that our universe only consists for four percent of ordinary matter. The remaining ninety-six percent is either dark energy or dark matter.
The existence of dark energy and dark matter is motivated by the fact that the amount of “visible” matter too low to explain the movement of far stars and flat galaxies consistently by our understanding of gravity and relativity. By postulating additional, invisible matter this problem is solved.
Though the existence of dark matter and dark energy is widely accepted among cosmologists, there is little agreement about what dark matter actually is. About the nature of dark matter, many different hypotheses have been proposed.
Since we do not know what dark matter is, we cannot rule out that dark matter is actually not matter. Again we stumble upon the (non-existent) definition of matter.
As long as there is no clarity about the nature of dark matter, I cannot say that I am a materialist.
The problem of consciousness is as old as mankind itself. And I believe that it will never be settled. Nevertheless this debate will continue until the last human will die.
Personally I am skeptical about the idea that we can reduce consciousness to a purely “materialistic” explanation. Of course, I could be wrong. However, I also reject idealism and (substance) dualism.
I am more comfortable with neutral monism. Simplistically we could describe this brand of monism as a midway between idealism and materialism. A more accurate description would be as follows.
According to neutral monism both the physical and the mental are just aspects of the same substance. Hence neutral monism is in direct contradiction to dualism which holds that the physical and the mental are different substances. Neutral monism is also distinct from both idealism and materialism/physicalism.
I admit this objection to materialism is largely a matter of feelings. Nevertheless, I will remind you that materialism has yet to produce a widely accepted explanation of consciousness. Maybe, we will find such explanation some time. Till then it will be good to consider alternative explanations as well.