What is empty space actually made of?

No single material is what empty space is made of. The phrase empty depends on what you have removed. Pump the air from a chamber and you get a vacuum, a region with very little gas. You have not removed space, physical fields or every trace of radiation.

Space between stars also contains scattered matter and light. It can be empty enough for some purposes without being absolute nothingness. This matters when a video moves from a statement about a vacuum to a claim about why anything exists. A physical vacuum is already part of a world with physical properties.

A field is a physical quantity described across space and time. A familiar magnetic field changes how a compass points. Modern particle theory uses quantum fields. Particles are excitations of those fields. An excitation is a state with more energy than the vacuum. Removing particles does not remove the framework that describes them.

The vacuum is the lowest-energy state in that framework. It has quantum properties that affect predicted measurements. Pictures of tiny particles appearing and disappearing can help introduce a calculation, but they should not be treated as a film of things that instruments have directly watched.

Cold is another word that needs care. The background microwave radiation has a temperature near 2.7 kelvin, a few degrees above absolute zero. Gas, dust and an object in sunlight can have different temperatures. Atoms in a laboratory can be cooled far below the background radiation’s temperature.

One deep puzzle remains. Vacuum energy should enter our account of gravity, yet theory does not naturally explain the small value associated with cosmic acceleration. That gap is real. It leaves an unanswered question about quantum fields and gravity, while laboratory tests still support many parts of quantum physics.