Do empty space in our universe exist or it is all filled with dark matter or some other unknown matter?
“Vacuum” is often loosely defined as the absence of matter.
Interstellar and intergalactic space has a very low density of atoms, so it can be considered a vacuum to a good approximation. However, every cubic centimeter of the universe actually contains about 400 photons left over from the Big Bang, so radiation fills the universe. There are also believed to be lots of leftover neutrinos everywhere, a kind of lightweight matter. Physicists also think dark energy (not dark matter) is everywhere. Dark matter is believed to mainly clump around galaxies, so there is probably not much of it between galaxies.
An ideal vacuum would have no matter (including no atoms, no neutrinos, and no dark matter) and no radiation (such as photons) either. But it could still have dark energy, which is sometimes called “vacuum energy”.
Physicists think of the ideal vacuum state as the state in which there are no field quanta but there can nevertheless be field energy.