So if space is a vacuum, how can we have dark matter? What does it contain that means that space still acts like a vacuum?
Dark matter is a hypothetical type of matter comprising approximately 27% of the mass and energy in the observable universe that is not accounted for by dark energy, baryonic matter (ordinary matter), and neutrinos. The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum. Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter, gravitational lensing, its influence on the universe's large-scale structure, and its effects in the cosmic microwave background. Dark matter is transparent to electromagnetic radiation and/or is so dense and small that it fails to absorb or emit enough radiation to be detectable with current imaging technology.
This isn't entirely correct. The vacuum state in quantum mechanics is stable and zero-temperature as long as there is no cosmological constant. It's just that in practice matter permeates our universe, such that there's no such thing of a perfect vacuum in our observable universe. Plus it looks like we have a non-zero cosmological constant.There is no such thing as a perfect vacuum. There isn't even in theory, quantum fluctuations don't allow it. Some packets of space are simply less dense than others, and what you consider dense it highly relative. To a neutrino, matter is just slightly polluted space.
If space is a vacuum, how can we have regular matter?So if space is a vacuum, how can we have dark matter?