What is not considered matter?

Not considered matter can refer to various things, but here are some examples:

1. Dark matter - This is a hypothetical type of matter that is believed to make up a large portion of the universe's mass but cannot be detected by current scientific instruments. It does not interact with light or other forms of electromagnetic radiation and only seems to exert gravitational forces on other matter.

2. Antimatter - This is a type of matter that has the opposite charge of normal matter, such as positrons instead of electrons. When antimatter comes into contact with normal matter, they annihilate each other and release energy.

3. Neutrinos - These are subatomic particles that have a very small mass and no electric charge. They can travel through matter without being affected by it, making them difficult to detect.

4. Virtual particles - These are subatomic particles that spontaneously appear in pairs for a short amount of time before disappearing again. They are not considered to be "real" particles but can have effects on other particles.

Overall, not considered matter often refers to particles or substances that behave in ways that are not currently fully understood or detectable with present technology.