If you're talking about physical substance:
* Immaterial: This implies something that lacks physical substance or form.
* Spiritual: This refers to the non-physical realm of the soul or spirit.
* Abstract: This describes concepts or ideas that are not concrete or tangible.
* Non-physical: This broadly indicates anything that isn't made of matter.
If you're talking about importance or consequence:
* Unimportant: This means something that doesn't have significance or value.
* Irrelevant: This describes something that is not related to the topic at hand.
* Insignificant: This indicates something that is small or unimportant.
* Trivial: This refers to something that is of little importance or consequence.
If you're talking about "matter" in the sense of "to be important":
* Not matter: This directly negates the importance of something.
* Doesn't matter: This implies that something is of no consequence.
Ultimately, the best antonym for "matter" depends on the specific context in which you're using it.