If something has mass it is matter.
They know because the effect the black hole has on other matter.
Plasma.
We do not even know that God exists, and this is just a matter of faith. We therefore can not 'know' that angels, the biblical messengers of God, exist.
All matter exists as either a solid, liquid, gas or plasma. This is called it's 'State Of Matter'. Oxygen exists as a gas.
The quantity of matter in an object is its mass. If you then know what the object is made from you can find out (approximately) how many atoms or molecules it has (measured in mol).
Plasma.
The absence of matter exists only in a vacuum. Modern science has revealed the exists of anti-matter. However, it could be debated on whether anti-matter is actually considered a form of matter, or the exact opposite of matter.
The region where no matter exists is called a vacuum or empty space. It is devoid of particles and has extremely low pressure and density.
matter
Matter
It is referred to as matter.
Plasma