Then it would depend upon what is meant by "matter". If matter was defined as "whatever exists" then it is trivially true.
If matter is defined as "made of atoms" then it is trivially false (neutron stars for example).
"Made of quarks" doesn't fare much better.
In this forum we have long searched for a working definition of matter and never found one that could gain consensus.
Generally positions like Materialism and Idealism are poorly defined.
Matter is defined by "conscious" materialists as a medium apart from consciousness that allows it to "arise", to "emerge".
Matter is an "abstraction" proposed by a "conscious reason".
It is impossible to be directly "in contact" with matter: to all extent and consequence, the world
is our
perceptions.
Once materialists got a "working definition" of matter they propose that their own consciousness (that which conceives, abstracts matter), must be reducible to matter -> in other words, to one of those abstractions of consciousness): and
that is a contradiction.
Matter is always defined quantitatively: mass, electric charge, frequency... So,
by definition, matter leaves qualitative experiences out —such as the redness of red, falling in love or the experience of listening Muddy Waters playing blues with his slide—.
For materialists it's all about
numbers. Quantities are fine to describe the differences between qualities (=measuring the wavelength of red vs. yellow, etc.) but they fail miserably when they try to describe the qualities: for it's own definition of matter lacks the explanatory power about
qualities.
The wavelengths of colour red
cannot absolutely describe the experience of that color to a blind man.
And that's where materialism completely fails, by replacing qualitative experiences with merely quantitative descriptions that cannot capture qualities.
So, we got that materialists "imagine" a world of matter separated from that very same "consciousness" that it is able to imagine a world of matter separated from that very same "consciousness"...
ad inphinitum.
Hence, materialism is fake.